Add 'roslyn/debuggerCompletion' to LSP

[jcouv]

This will allow supporting IntelliSense completion in debugger expression windows (Quick Watch, Watch, Immediate) via the debug adapter and Roslyn LSP. This improves the debugging experience when using the C# Dev Kit extension for VS Code.

Key changes:

• add roslyn/debuggerCompletion handler
• extract and re-use splicing logic
• a couple of subtleties:
  • set specific completion options (mainly so that out-of-scope locals aren't filter out)
  • adjust the position on returned completion items (so that subsequent calls to resolve match the unspliced document)

Open issue: confirm that the debug adapter has access to LSP

[jcouv]

📝 Extracted from CSharpDebuggerIntelliSenseContext.GetAdjustedBuffer