Disable non syntax errors in files that don't have semantic capabilities

[mjbvz]

If we're on a host that supports semantic checking, but in a file that doesn't support semantic errors, we currently don't want to report semantic errors since these will likely complain about imports and other undefined symols

[mjbvz]

@DanielRosenwasser Interested to see if there's a better way to fix this

The basic problem is:

1. On Desktop, VS Code will try to spawn a syntax and semantic TS server
2. We are working in an in-memory workspace that TS cannot read. All files are opened as in-memory resources
3. When calling geterr for these files, we go to the semantic server
4. However the semantic server then ends up treating these files like there were part of a real workspace. It tries resolving import paths

We could fix this by calling geterr on the syntax server, but for cases where you have a mix of on-disk and in-memory files, we'd have to make two geterr calls

The problem with my fix is that TS is still computing the diagnostics but we are just discarding them. It would be better if we never computed these in the first place

[DanielRosenwasser]

I think I need to better understand the use-cases - we expanded the partial semantic server to support diagnostics for situations where we know that a full semantic server can't be used.