ast-check for zls!

[SuperAuguste]

This should be ready to merge!

Small question I have is whether or not we should be emitting tree.errors diagnostics when ast-check diagnostics are enabled; they're usually the same (excluding stuff that only astgen can catch, of course) and it leads to error doubling (are there even cases where these would be different?)

[Techatrix]

After taking a quick look into the source code of ast-check you can see here that tree.errors gets also printed as part of ast-check. Which explains why some diagnostics are very similar.

Therefore we have to options so that we avoid duplicate diagnostics:

• Remove this code and let ast-check handle everything.
• Only run and report ast-check if tree.errors is empty (i would recommend this option)

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Also setup.zig should be updated according to the config changes.