feat: migrate the delta encoder

[LucasXu0]

closes #273

Preview

Code sample

const json = r'''[{"insert":"Hello AppFlowy!"},{"attributes":{"header":1},"insert":"\n"}]''';
final delta = Delta.fromJson(jsonDecode(json));
final document = quillDeltaEncoder.convert(delta);
final editorState = EditorState(document: document);

Documentation

https://github.com/AppFlowy-IO/appflowy-editor/blob/6577d212e80c8765da51ce5addf65a774ea95828/documentation/importing.md

[codecov]

Codecov Report

Merging #277 (a9eb307) into main (8bfb37f) will increase coverage by 1.02%.
The diff coverage is 99.13%.

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
+ Coverage   62.61%   63.64%   +1.02%     
==========================================
  Files         246      246              
  Lines       10857    10864       +7     
==========================================
+ Hits         6798     6914     +116     
+ Misses       4059     3950     -109     

Impacted Files	Coverage Δ
...b/src/plugins/quill_delta/quill_delta_encoder.dart	99.13% <99.13%> (ø)

... and 1 file with indirect coverage changes