Better types remove outdated syntax

[7heMech]

In this pr:

• I add return types and add more jsdoc
• I replaced outdated substr syntax

[7heMech]

It also improves build times

[bellstrand]

As I wrote in your issue, the bun part has a couple of more moving parts that would need to be resolved first. (dependabot, test/release workflows etc). and Dependabot would need to be replaced since they don't support bun yet, dependabot/dependabot-core#6528

So if you could move all things related to bun to another PR, we can merge the other things right away! :)

[7heMech]

As I wrote in your issue, the bun part has a couple of more moving parts that would need to be resolved first. (dependabot, test/release workflows etc). and Dependabot would need to be replaced since they don't support bun yet, dependabot/dependabot-core#6528

So if you could move all things related to bun to another PR, we can merge the other things right away! :)

Fair enough bun will have more compatibility with jest in the future, so we could even avoid modifying the tests a lot.

[7heMech]

I think these are the changes not related to bun.

[7heMech]

Actually, I'll close this since everything is in my other pr I think