If a package uses only `.js` then supply `--extensions ".js"`. If a package
uses only `.ts` then supply `--extensions ".ts"`. If a package uses both, then
supply `--extensions ".js,.ts"`.
The reason for this is that adding/removing TS/JS support ought to occasion
revising a package's `package.json` file and adjusting other scripts as well,
e.g. for linting. With these changes, it won't work to simply start adding
`.ts` files in a package's `src/` directory, which should prompt the developer
to review `package.json` and make the appropriate changes, and/or other
developers may realize changes need to be made during code review.
Michael Bradley, Jr
Pascal Precht