Document to PDF feature for AppFlowy Editor

[rileyhawk1417]

This PR is an initial draft for the feature to provide a built-in converter that can convert from Markdown to PDF. The feature is using some code from htmltopdfwidgets as reference.

Checklist

• [x] Support Checkboxes.
• [x] Support Images in document.
• [ ] Coloured text/Highlighted text.
• [x] Render Tables(partially supported)

Whats supported

• [x] Basic Text
• [x] Bullet Lists.
• [x] Todo List/Checklist
• [x] Emoji Fonts.

The todo items may change overtime depending on the requirements of the feature. When its ready will mark this PR as ready.

[rileyhawk1417]

This PR is meant to target Document type for exports, also addressing the following issue #3264 #2938

[rileyhawk1417]

@LucasXu0 & @Xazin am working on table support now, will mark it as ready when am done. Then open another PR for writing the tests.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 303 lines in your changes missing coverage. Please review.

Project coverage is 73.24%. Comparing base (37432ea) to head (77a3bf4). Report is 1 commits behind head on main.

Files	Patch %	Lines
lib/src/plugins/pdf/html_to_pdf_encoder.dart	0.00%	275 Missing :warning:
lib/src/plugins/pdf/extension/color_ext.dart	0.00%	28 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #695      +/-   ##
==========================================
- Coverage   74.80%   73.24%   -1.56%     
==========================================
  Files         313      315       +2     
  Lines       14264    14567     +303     
==========================================
  Hits        10670    10670              
- Misses       3594     3897     +303     

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[rileyhawk1417]

Have added table support. Will open another PR to handle the tests.