Add CompletionWindow Open/Close callbacks

[waf]

Fixes #272 (feature request)

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.5%. Comparing base (8e15877) to head (66dae2a).

Files with missing lines	Patch %	Lines
src/PrettyPrompt/PromptCallbacks.cs	80.0%	0 Missing and 2 partials ⚠️
src/PrettyPrompt/Panes/CompletionPane.cs	92.3%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #293   +/-   ##
=====================================
  Coverage   84.4%   84.5%           
=====================================
  Files         47      47           
  Lines       3952    3972   +20     
  Branches     660     664    +4     
=====================================
+ Hits        3339    3357   +18     
  Misses       452     452           
- Partials     161     163    +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.