Build: consider getting rid of inline tasks in the build

[KirillOsenkov]

They use very slow CodeDOM for code generation.

[kzu]

We could just switch to the built-in Roslyn one and it would be awesome automatically?

[KirillOsenkov]

I’ve later discovered that the Roslyn one has its problems (It was not caching properly, so doing expensive work every time). So maybe write a task .dll by hand or just leave it alone if you’re happy with perf. Or let’s fix the Roslyn one and use it then :)

[kzu]

Switched to source generators 💪