feat: add 'delete line' command shortcut

[LucasXu0]

Press Cmd (on Mac) or Ctrl (on Windows/Linux) + X will:

• If the selection is collapsed, delete the current line.
• If the selection is not collapsed, do nothing.

related to AppFlowy-IO/AppFlowy#6425

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.89%. Comparing base (d049584) to head (19ec996).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._component/base_component/delete_line_command.dart	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #906      +/-   ##
==========================================
+ Coverage   71.87%   71.89%   +0.02%     
==========================================
  Files         318      319       +1     
  Lines       14943    14959      +16     
==========================================
+ Hits        10740    10755      +15     
- Misses       4203     4204       +1     

Flag	Coverage Δ
	71.89% <93.75%> (+0.02%)	⬆️

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