Convert loot-core to TS p1 (#841)

Part 1 of the conversion. Mostly renaming js to ts and making sure
things make still sense. Added also handy TS ESLint rules.

In order to support the various .web/.electron/... I ended up adopting
`index.d.ts` as pattern to share type definition. Let me know if that
makes sense for you too. Right now the function type definition is
duplicated, but the solution will be importing from `index.d.ts` and
using `const fn: FnDef = () => ...` that way we can keep all variants in
sync from a single type file.

Such rewrite however is better done in another PR otherwise we risk
confusing git and loosing history (rename + too many changes). Another
thing that might do in the next PR is convert all files to ESModules, as
things get confusing between CJS exports, ESM default/named and TS adds
extra complains.