Support complex completions in roslyn (e.g. casting)

[waf]

See #97

It required changes in PrettyPrompt, which are now complete in PrettyPrompt v6.0.4 -- so this PR now consumes those changes and hooks it up to Roslyn's completion API.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.4%. Comparing base (f9008b9) to head (690c7a8).

Files with missing lines	Patch %	Lines
...arpRepl.Services/Completion/AutoCompleteService.cs	69.2%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #507     +/-   ##
=======================================
- Coverage   80.5%   80.4%   -0.1%     
=======================================
  Files        130     130             
  Lines       9481    9502     +21     
  Branches    1427    1430      +3     
=======================================
+ Hits        7633    7642      +9     
- Misses      1356    1366     +10     
- Partials     492     494      +2     

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