optim(cache): don't check imports for syntactic diagnostics

[agilgur5]

Summary

Syntactic diagnostics are only per-file, so we don't need to checkImports in the cache's isDirty check for them

Details

• Per the TS LS Wiki, syntactic diagnostics only need to parse the specific file in question
  • since syntax is independent of imports; imports only affect semantics

References

• Just happened to finally realize this while reading the LS Wiki for optim(watch): don't reset DocumentRegistry b/t watch cycles #388 .
• Related to fix(cache): invalidate codeCache in most cases when imports change #369 ; that was a de-opt for codeCache, this is an optimization for syntacticDiagnosticsCache