chore: support for shift+home/end keys

[MayurSMahajan]

Partially Solves: #2888

In the current AppFlowy Editor version the shortcut we support for selecting till the beginning of a line and till the end of a line is Ctrl/Cmd + shift + arrow left/arrow right. Although in Windows and Linux the default shortcut for this is Shift+home/end. This PR enables that.

Another default behavior on Windows and Linux is selecting a word with Ctrl+arrow left/arrow right, however, the current editor version achieves the same functionality by alt+arrow left/arrow right. This PR introduces the default command for Windows and Linux.

This PR also contains some new tests that improve the code coverage.

[codecov]

Codecov Report

Merging #306 (681cd1d) into main (d87d353) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #306   +/-   ##
=======================================
  Coverage   74.53%   74.53%           
=======================================
  Files         237      237           
  Lines        9519     9519           
=======================================
  Hits         7095     7095           
  Misses       2424     2424           

Impacted Files	Coverage Δ
...ts/command_shortcut_events/arrow_left_command.dart	92.53% <ø> (ø)
...s/command_shortcut_events/arrow_right_command.dart	91.93% <ø> (ø)