flow: add type definitions for better-sqlite3 (#837)

Summary:
The `flow-typed` repository doesn’t have type definitions for
`better-sqlite3`, so I wrote some. I figure that we can use them for a
while, fix any problems that arise, and then PR them upstream.

I started from [the TypeScript definitions][1], but made some
improvements (like stronger typing for bound parameters), and also made
some necessary changes due to differences between Flow and TypeScript.

[1]: https://github.com/DefinitelyTyped/DefinitelyTyped/commits/master/types/better-sqlite3/index.d.ts

Prettier does not format this file (it is in `flow-typed`), so I
manually ran it through Prettier with the settings used by `flow-typed`
itself.

Test Plan:
None.

wchargin-branch: flow-better-sqlite3

flow-typed/npm/better-sqlite3_vx.x.x.js

@@ -0,0 +1,139 @@
+// flow-typed signature: 946429b216273f6ed9345df0294cfd25
+// flow-typed version: <<STUB>>/better-sqlite3_v4.1.4/flow_v0.77.0
+
+declare class bettersqlite3$Database {
+  +memory: boolean;
+  +readonly: boolean;
+  +name: string;
+  +open: boolean;
+  +inTransaction: boolean;
+
+  constructor(
+    filename: string,
+    options?: bettersqlite3$Database$ConstructorOptions
+  ): void;
+  prepare(source: string): bettersqlite3$Statement;
+  exec(source: string): this;
+  transaction(sources: $ReadOnlyArray<string>): bettersqlite3$Transaction;
+  pragma(pragma: string, simplify?: boolean): any;
+  checkpoint(databaseName?: string): this;
+  register(fn: (...args: any[]) => any): void;
+  register(
+    options: bettersqlite3$Database$RegisterOptions,
+    fn: (...args: any[]) => any
+  ): void;
+  close(): this;
+  defaultSafeIntegers(toggleState?: boolean): this;
+
+  static SqliteError: Class<bettersqlite3$SqliteError>;
+}
+
+declare type bettersqlite3$Database$ConstructorOptions = {
+  +memory?: boolean,
+  +readonly?: boolean,
+  +fileMustExist?: boolean
+};
+
+declare type bettersqlite3$Database$RegisterOptions = {
+  +name?: string,
+  +varargs?: boolean,
+  +deterministic?: boolean,
+  +safeIntegers?: boolean
+};
+
+// For functions that accept bound parameters, we permit an optional
+// binding dictionary, followed by zero or more bound values. In
+// reality, better-sqlite3 is more flexible than this: the binding
+// dictionary can go anywhere among the arguments. We can't express this
+// type, though, and it is strange to use both named and numbered
+// parameters at all, so we accept this concession.
+//
+// Also note that better-sqlite3 permits binding `Integer.IntLike` from
+// npm/integer, not just `number`, but we don't have those typedefs. For
+// now, `number` is a good approximation.
+declare type bettersqlite3$BoundValue = number | string | Buffer | null;
+declare type bettersqlite3$BindingDictionary = {
+  [string]: bettersqlite3$BoundValue
+};
+
+declare class bettersqlite3$Statement {
+  +memory: boolean;
+  +readonly: boolean;
+  +name: string;
+  +open: boolean;
+  +inTransaction: boolean;
+
+  run(...params: bettersqlite3$BoundValue[]): bettersqlite3$RunResult;
+  run(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): bettersqlite3$RunResult;
+  get(...params: bettersqlite3$BoundValue[]): any;
+  get(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): any;
+  all(...params: bettersqlite3$BoundValue[]): any[];
+  all(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): any[];
+  each(params: any, cb: (row: any) => void): void;
+  each(cb: (row: any) => void): void;
+  each(...params: bettersqlite3$BoundValue[]): void;
+  each(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): void;
+  pluck(toggleState?: boolean): this;
+  bind(...params: bettersqlite3$BoundValue[]): this;
+  bind(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): this;
+  safeIntegers(toggleState?: boolean): this;
+}
+
+declare class bettersqlite3$Transaction {
+  +database: bettersqlite3$Database;
+  +source: string;
+
+  constructor(db: bettersqlite3$Database, sources: string[]): void;
+  run(...params: any[]): bettersqlite3$RunResult;
+  run(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): bettersqlite3$RunResult;
+  bind(...params: any[]): this;
+  bind(
+    namedParams: bettersqlite3$BindingDictionary,
+    ...params: bettersqlite3$BoundValue[]
+  ): this;
+  safeIntegers(toggleState?: boolean): this;
+}
+
+declare interface bettersqlite3$RunResult {
+  changes: number;
+  // TODO: This is actually `Integer.IntLike` from npm/integer, but we
+  // don't have those typedefs. For now, `number` is a good
+  // approximation.
+  lastInsertROWID: number;
+}
+
+declare class bettersqlite3$SqliteError extends Error {
+  +code: string;
+  constructor(message: string, code: string): void;
+}
+
+declare module "better-sqlite3" {
+  declare export type Database = bettersqlite3$Database;
+  declare export type Database$ConstructorOptions = bettersqlite3$Database$ConstructorOptions;
+  declare export type Database$RegisterOptions = bettersqlite3$Database$RegisterOptions;
+  declare export type BoundValue = bettersqlite3$BoundValue;
+  declare export type BidningDictionary = bettersqlite3$BindingDictionary;
+  declare export type Statement = bettersqlite3$Statement;
+  declare export type Transaction = bettersqlite3$Transaction;
+  declare export type RunResult = bettersqlite3$RunResult;
+  declare export type SqliteError = bettersqlite3$SqliteError;
+  declare module.exports: Class<bettersqlite3$Database>;
+}