feat: typescript 3 -> 4

[FauxFaux]

Move from compiling on typescript 3.9 to typescript 4.3.

This is annoying me as it means we end up with a production dependency of 3.9, instead of the 4.3 we're using locally. I have no idea how this is ending up as a production dependency, but it is! Thanks yarn. Tharn.

Note that typescript aren't great at semver; 3.8 -> 3.9 is the same level as change as 3.9 -> 4.0 and 4.1 -> 4.2, etc.

That is, this isn't a huge breaking change.