fix: improves cursor left word delete

[MayurSMahajan]

Solves: #87
This PR makes sure that by pressing ctrl+backspace in the editor, now also deletes the whitespace that it encounters before a word.

For given sentence: an apple |
the actual behavior is an apple|
but the expected behavior should be: an |

This PR achieves the expected behavior.

Note:

1. This PR moves the cursorLeftWordDelete and cursorLeftSentenceDelete handlers in a new file (previously stored in arrow_keys_handler.dart ).
2. This PR moves the extension methods written in arrow_keys_handler.dart file to a new file.
3. Selection Range enum is now made public.

[codecov]

Codecov Report

Merging #88 (86f1edc) into main (0f303cb) will increase coverage by 0.07%.
The diff coverage is 96.34%.

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   86.79%   86.86%   +0.07%     
==========================================
  Files         124      127       +3     
  Lines        7322     7369      +47     
==========================================
+ Hits         6355     6401      +46     
- Misses        967      968       +1     

Impacted Files	Coverage Δ
...rvice/shortcut_event/built_in_shortcut_events.dart	98.19% <ø> (+0.03%)	⬆️
...nternal_key_event_handlers/arrow_keys_handler.dart	99.40% <92.30%> (+1.06%)	⬆️
lib/src/extensions/position_extension.dart	94.59% <94.59%> (ø)
...key_event_handlers/cursor_left_delete_handler.dart	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes

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[Xazin]

Lgtm