Convert the bin entry to TypeScript

[remcohaszing]

This means it’s type checked. There’s no need to have this particular file as JavaScript.

[eemeli]

Why? There's no benefit to this change. Note how even the updated contents are valid JS.

[remcohaszing]

I don’t see a reason to treat the bin entry different from any other part of the source code. With this change, the file is both type checked and linted.

This just seemed like a small improvement I noticed while looking at the code base.

[eemeli]

The bin.js file needs to be executable. I find it much cleaner to have that be always set, rather than needing to add a post-build step for it (which this PR is currently missing, btw).