fix: pass documentRegistry and serverMode to createLanguageService

[ekazakov14]

This adds caching between different monorepository projects

Summary

ts.createLanguageService was called with only the host argument, missing
documentRegistry and languageServiceMode:

• documentRegistry — without it, the plugin creates an isolated document
  registry instead of sharing the one managed by ProjectService. When multiple
  projects include overlapping files, each gets its own parsed SourceFile
  copies, increasing memory usage.
• serverMode — without it, the service defaults to full semantic mode
  regardless of the mode the TS server was started with.

Both values are now forwarded from the original ProjectService instance.

Notes

ProjectService.documentRegistry exists at runtime but is typed as private
in TypeScript's declarations, so a type cast is required.