Automatic type definition file

[rj-au]

TL;DR
Would it be worth switching to TS first files, so the definition file isn't manually maintained?

Long read:
Would it be worth renaming the .js files to .ts and using tsup to transpile to cjs and esm, and adding exports for both in the package.json?

This way you won't need to maintain types manually in the .d.ts, definition file, and the imports for users is automatic.

Related to this PR: #47

[liammclennan]

Hi Richard,

I don't know anything about tsup, but converting the library to Typescript seems like an improvement. If it is possible to do it with just Typescript (i.e. without adding a new dependency) that is our preference. If you are interested in creating a PR we'd be happy to look at it. Thanks!!