Upgrade to TypeScript 6 #4486

Is your feature request related to a problem? Please describe.
Upgrade to compile with TypeScript 6. This is non-trivial, since TypeScript 6 turns on some compiler options by default that used to be off by default.

Describe the solution you'd like
Update to TypeScript 6 and adjust the tsconfig.json and sources so that the code still compiles.

Describe alternatives you've considered
Not doing it: we'd fall behind over time.

Additional context
From what I understand it'll make adapting to ES Modules easier.