Add superscript and subscript

[Amir-P]

Fixes #273 .

Known issues:

• Capital letters are not rendered correctly with Roboto font (Maybe related to Font Feature not applied correctly in Rich Text flutter/flutter#107527)
• Not working on Web (Maybe related to Setting FontFeature has no effect on web flutter/flutter#64595)

[codecov]

Codecov Report

Attention: Patch coverage is 64.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 87.67%. Comparing base (eeac387) to head (071825f).
Report is 36 commits behind head on master.

Files	Patch %	Lines
...ackages/parchment/lib/src/document/attributes.dart	22.22%	7 Missing ⚠️
packages/fleather/lib/src/widgets/text_line.dart	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
- Coverage   87.72%   87.67%   -0.06%     
==========================================
  Files          64       64              
  Lines       10413    10438      +25     
==========================================
+ Hits         9135     9151      +16     
- Misses       1278     1287       +9     

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