feat(dev): add assertUnreachable

Asserts that a case is unreachable in `switch` statements (and really
any flow control construct)

src/utilities.ts

@@ -0,0 +1,18 @@
+/**
+ * Asserts that a case in a switch statement is unreachable.
+ *
+ * @example
+ * // Exhaustively checks `user.role` type ("admin" | "user") and will not compile
+ * // if a new role is added without adding a case to the switch statement.
+ * switch (user.role) {
+ *    case "admin":
+ *       break;
+ *    case "user":
+ *      break;
+ *    default:
+ *      assertUnreachable(user.role);
+ * }
+ */
+export function assertUnreachable(value: never): never {
+    throw new Error(`Unreachable case: ${value}`);
+}