Support complex completions that rewrite the source document

[waf]

Support complex completions, where "complex" means they rewrite the source document rather than just inserting the selected completion text.

This will be used to fix waf/CSharpRepl#97

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.6%. Comparing base (7dd3051) to head (9efaba4).

Files with missing lines	Patch %	Lines
src/PrettyPrompt/Completion/CompletionEdit.cs	80.0%	0 Missing and 1 partial ⚠️
src/PrettyPrompt/Completion/CompletionItem.cs	85.7%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #304     +/-   ##
=======================================
+ Coverage   86.5%   86.6%   +0.1%     
=======================================
  Files         47      48      +1     
  Lines       4096    4116     +20     
  Branches     679     683      +4     
=======================================
+ Hits        3546    3568     +22     
+ Misses       384     381      -3     
- Partials     166     167      +1     

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