feat(fleather): add iPad Scribble support

[zhpoo]

Changes:

• expose stylusHandwritingEnabled from FleatherEditor, FleatherField, and RawEditor;
• implement Flutter's ScribbleClient lifecycle for editable iOS editors;
• report UTF-16 character, caret, composing, size, and transform geometry to the text input connection;
• render and remove Scribble placeholders without changing the Parchment document;
• support selection boxes across Fleather block nodes;
• accept Scribble handwriting across the full editor surface without moving the current selection on pen-down;
• connect Fleather history to the iOS system undo manager for Pencil toolbar undo and redo;
• reject Scribble input whose center is outside the editor bounds;
• keep Scribble disabled at runtime on non-iOS platforms and in read-only editors.

[codecov]

Codecov Report

❌ Patch coverage is 93.58491% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.23%. Comparing base (1ee8bdc) to head (10aeb47).

Files with missing lines	Patch %	Lines
packages/fleather/lib/src/widgets/editor.dart	92.53%	10 Missing ⚠️
packages/fleather/lib/src/widgets/text_line.dart	84.21%	6 Missing ⚠️
packages/fleather/lib/src/widgets/history.dart	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage   88.11%   88.23%   +0.12%     
==========================================
  Files          63       63              
  Lines       10899    11156     +257     
==========================================
+ Hits         9604     9844     +240     
- Misses       1295     1312      +17     

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[Amir-P]

Thanks for working on this! I took a look at the changes and left some comments. Please take a look. @zhpoo

I don't own an iPad so I'm not gonna be able to test and verify the behavior. Will you be able to do that? @amantoux

[Amir-P]

I believe we should skip the check for iOS platform here. If we are the client, we should update the state anyway.

[Amir-P]

I don't see a reason for introducing a timer and periodically updating the undo state as the undo state is retrieved from the controller and it would notify the listeners if the state changes. Am I missing something?

[Amir-P]

I think we should just use the renderEditor.size here and not take padding or content width into account as user should be able to write with stylus anywhere within the editor area. This is also what Flutter's EditableText does:

  void _updateSizeAndTransform() {
    final Size size = renderEditable.size;
    final Matrix4 transform = renderEditable.getTransformTo(null);
    _textInputConnection!.setEditableSizeAndTransform(size, transform);
  }

[Amir-P]

I think it's better to make the focus node required and non-optional for the RawEditor now that we have lifted the internal focus node creation up. It's gonna be a breaking change but I don't think many would be affected. WDYT? @amantoux