Make completion cancellable and handle REPL keywords

[Rekkonnect]

• RoslynServices.CompleteAsync can be cancelled with CancellationToken, as provided by GetCompletionItemsAsync
• REPL command keywords (help, exit, clear)
  • Highlight them in the prompt to show that they will be executed, matching the behavior of the command handler
  • Show them as completion items

[codecov]

Codecov Report

❌ Patch coverage is 95.06173% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.3%. Comparing base (9c0204b) to head (4bdf1b9).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
CSharpRepl/CSharpReplPromptCallbacks.cs	95.0%	2 Missing and 1 partial ⚠️
CSharpRepl/ReadEvalPrintLoop.cs	94.4%	0 Missing and 1 partial ⚠️

❌ Your project status has failed because the head coverage (67.3%) is below the target coverage (70.0%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #403     +/-   ##
=======================================
+ Coverage   67.1%   67.3%   +0.2%     
=======================================
  Files         90      90             
  Lines       6748    6813     +65     
  Branches     930     932      +2     
=======================================
+ Hits        4528    4591     +63     
- Misses      1896    1899      +3     
+ Partials     324     323      -1     

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[waf]

Thanks!