feat: export document to PDF (#695)

* feat: create initial parser for PDF format

* chore: add comments to explain code.

* chore: commit missing file

* feat: init commit for pdf parser

* chore: bind dependancy to git repo fork for fixes

* feat: get basic text from parser

* feat: get textstyle working with various texts

* fix: ensure formatting(bold, italics) doesnt break paragraphs

- Remove additional comments.

* chore: remove htmltopdfwidgets dependancy

* chore: clean up redundant code and add helper functions

* chore: update example files to include pdf option for export

* fix: remove html exportFileType

* feat: add in image support

* feat: add image support for network & local image

* feat: enable table support

* fix: enable tables in pdf parser

- Tables are enabled but lists within a table are not working yet.
- There is also the case of strikethrough not working, probably it didnt get rendered as a tag.
- Cleaned up example.dart

* chore: clean up comments in files & unused imports.

* chore: remove unused file.

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Co-authored-by: Lucas.Xu <lucas.xu@appflowy.io>

pubspec.yaml

@@ -46,6 +46,9 @@ dependencies:
   string_validator: ^1.0.0
   tuple: ^2.0.1
   universal_html: ^2.2.4
+  pdf: ^3.10.7
+  http: ^1.2.0
+  printing: ^5.11.1
   url_launcher: ^6.1.11
   visibility_detector: ^0.4.0+2