fix: support character deletion while holding down the Shift key.

[Jayaprakash-dev]

This fix is related to the bug in AppFlowy-IO/AppFlowy:

[Bug] Shift blocks backspace deletion # 4279
[link]: AppFlowy-IO/AppFlowy#4279

I've addressed the reported issue with the following changes:

Issue:
When the shift key is held down, deleting a character or selection by pressing backspace is not functioning as expected.

Solution:
I've implemented a fix by assigning the same Backspace handler in backspace_command.dart to another command shortcut event handler named deleteLeftCharacterCommand which is responsible for handling shift+backspace shortcut event. This adjustment ensures that the deletion functionality works seamlessly, resolving the reported bug.

Repository Context:
Although the issue is posted in another related repository AppFlowy-IO/AppFlowy, the code files relevant to this bug are present in this repository. Therefore, I've created this PR here.

Additional Changes:
I've also included the required test files to ensure the reliability of this fix.

[CLAassistant]

All committers have signed the CLA.

[LucasXu0]

It's not necessary to define a new shortcut command for Shift+Backspace. Simply adding this shortcut (Shift+Backspace) to the existing Backspace command is enough.

[Jayaprakash-dev]

@LucasXu0, Just got to know, that new shortcuts can be added to existing command handlers through a comma-separated string. As a result, I removed the unnecessary deleteLeftCharacterCommand handler along with its unit tests. However, I retained the widget tests for the shift+backspace command event.

Thanks for the clarification!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (87eabfb) 76.50% compared to head (2856714) 76.50%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #659   +/-   ##
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  Coverage   76.50%   76.50%           
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  Files         295      295           
  Lines       13039    13039           
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  Hits         9975     9975           
  Misses       3064     3064           

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