Support block delete with word and line modifiers - #79695

This PR focuses on supporting modifiers to delete blocks of text in a editable widget.

Shortcuts per OS

• macOS
  • CMD (⌘) + Backspace: Delete all words left to the cursor
  • Option + Backspace: Delete word left to the cursor
• Non macOS
  • Alt + Backspace: Delete all words left to the cursor
  • Ctrl + Backspace: Delete word left to the cursor

Some cases for reference:

case: 'text with| multiple words'
word modifier + backspace: 'text | multiple words'
line modifier + backspace: '| multiple words'
word modifier + delete: 'text with| words'
line modifier + delete: 'text with|'

case: 'text with multiple\n|words'
word modifier + backspace: 'text with |words'
line modifier + backspace: 'text with multiple\n|words'
word modifier + delete: 'text with multiple\n|'
line modifier + delete: 'text with multiple\n|'

case: 'text with multiple|\nwords'
word modifier + backspace: 'text with |\nwords'
line modifier + backspace: '|\nwords'
word modifier + delete: 'text with multiple|'
line modifier + delete: 'text with multiple|\nwords'

case: 'text with\n|\nmultiple words' in obscure text
word modifier + backspace: '|\nmultiple words'
line modifier + backspace: '|\nmultiple words'
word modifier + delete: 'text with\n|'
line modifier + delete: 'text with\n|'

case: 用多個塊測試 (cursor at offset 4)
backspace: '用多個測試' (cursor at offset 3)
word modifier + backspace: '用多個測試' (cursor at offset 3)
line modifier + backspace: '測試' (cursor at offset 0)
word modifier + delete: '用多個塊試' (cursor at offset 4)
line modifier + delete: '用多個塊' (cursor at offset 4)

case: برنامج أهلا بالعالم (cursor at offset 4 - from the right)
backspace: برنمج أهلا بالعالم (cursor at offset 3 - from the right)
word modifier + backspace: مج أهلا بالعالم (cursor at offset 0 - from the right)
word modifier + delete: برنا أهلا بالعالم (cursor at offset 4 - from the right)

Related issue:

• Fixes [macOS] Textfields should support CMD (⌘) + Backspace and Control + Backspace #57015
• Fixes Delete by word does not work #77547

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.

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