Bump TypeScript version

[benmccann]

Just a bit of housekeeping. Sticking with the 3.x series for now

[Conduitry]

I haven't looked into where the dependency is coming from, but is it possible to also update tsutils so that we don't need to have two versions of tslib?

[dummdidumm]

TypeScript does not follow semver, there are breaking changes in-between or new type features. We would need to look into what's changed between our old and the new version to make sure we cannot accidentally add new type features that would break people. That said, even if we bump to the latest version, I doubt we will do that much type ninja stuff that it would really matter.

Edit: just read that we need 3.7+ for optional chaining to work. I'm ok to bump to that version if it makes writing code easier.
When bumping we should check the new generated js and dts output to look the same as previously.
Things to watch out for: https://devblogs.microsoft.com/typescript/announcing-typescript-3-6/#get-and-set-accessors-allowed-in-ambient-contexts
Don't use https://devblogs.microsoft.com/typescript/announcing-typescript-3-7/#assertion-functions
Don't do recursive types https://devblogs.microsoft.com/typescript/announcing-typescript-3-7/#more-recursive-type-aliases

[benmccann]

There's a PR pending up update tsutils, but that library looks abandoned and hasn't had any updates in over a year

[benmccann]

@dummdidumm I just noticed your edit, so changed it to 3.7 per your suggestion