breaking: Remove SelectableHtml

[Sub6Resources]

This should resolve a few issues regarding the SelectableHtml/Html feature parity. Users can achieve a much cleaner result by using a SelectionArea widget to wrap Html.

Fixes #1137
Closes #717 Closes #1134 Closes #1105

[Sub6Resources]

@erickok have you had a chance to look at this PR?

[erickok]

To be honest, since we're still trying to get a 3.0 ready, breaking the API big time, I'd get rid of the limited SelectableHtml entirely. Deprecations are nice in minor releases and are developer friendly, but we have a clean, superior and 'free' alternative.

[Sub6Resources]

That works for me! I'll get this updated sometime today

[codecov]

Codecov Report

Base: 51.40% // Head: 51.07% // Decreases project coverage by -0.32% ⚠️

Coverage data is based on head (b067c71) compared to base (c75e0df).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1153      +/-   ##
==========================================
- Coverage   51.40%   51.07%   -0.33%     
==========================================
  Files          19       19              
  Lines        2640     2561      -79     
==========================================
- Hits         1357     1308      -49     
+ Misses       1283     1253      -30     

Impacted Files	Coverage Δ
lib/custom_render.dart	53.02% <ø> (+2.05%)	⬆️
lib/flutter_html.dart	65.30% <ø> (-4.02%)	⬇️
lib/html_parser.dart	84.98% <ø> (+1.01%)	⬆️
lib/src/css_box_widget.dart	63.17% <100.00%> (-1.00%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[Sub6Resources]

@erickok. It's all removed!

[erickok]

Just a README update is needed.

[erickok]

This should be updated now to reflect it's removal rather than deprecation.

[Sub6Resources]

As I've been (finally) working at the migration from customRender to Extension, I ended up touching a lot of the same code as this pull request. As such, I decided to just include all the changes from this PR into the other branch manually rather than resolving massive merge conflicts after the fact. That said, I'm going to close this PR, with the changes continuing on into #1176. I'll push the changes to that PR in a few hours.