Use bun to run tests, thus supporting Typescript for testing

[tomlarkworthy]

This does not adopt TS in the core yet but we can now start to use TS in tests without affecting anything. Using TS in prod code e.g. index.js is a problem because the package.json exports the index.js as the entrance point to the module, so if we remove index.js then we will need a bundle step to convert he TS source files to a JS file for use in npm publishing. Code used exclusively by tests does not have this problem though, so this is a good start.

[markusahlstrand]

Looks good. For whatever reason the build hasn't started. I'll merge this and have a look