Allow Alt+Enter to insert soft newline

[waf]

Claude Code remaps Shift+Enter to Alt+Enter in Windows Terminal. That has the side effect of breaking Shift+Enter in PrettyPrompt/CSharpRepl.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.0%. Comparing base (cc3f42a) to head (b337ce2).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #281   +/-   ##
=====================================
  Coverage   85.0%   85.0%           
=====================================
  Files         46      46           
  Lines       3943    3943           
  Branches     603     603           
=====================================
+ Hits        3353    3354    +1     
  Misses       448     448           
+ Partials     142     141    -1     

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