feat: Update CssBoxWidget to handle rtl marker boxes

[Sub6Resources]

This involved adding more options to the Margins class, as well as refactoring the padding option to take a new HtmlPaddings object.

Fixes #1257

[codecov]

Codecov Report

Patch coverage: 73.68% and project coverage change: +4.09 🎉

Comparison is base (496d1aa) 60.02% compared to head (103ce62) 64.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1270      +/-   ##
==========================================
+ Coverage   60.02%   64.11%   +4.09%     
==========================================
  Files          37       38       +1     
  Lines        2699     2979     +280     
==========================================
+ Hits         1620     1910     +290     
+ Misses       1079     1069      -10     

Impacted Files	Coverage Δ
lib/src/style/padding.dart	51.02% <51.02%> (ø)
lib/src/style/margin.dart	64.00% <61.44%> (-19.79%)	⬇️
lib/src/style.dart	85.64% <76.92%> (+0.84%)	⬆️
lib/src/css_parser.dart	35.92% <89.92%> (+18.44%)	⬆️
lib/src/css_box_widget.dart	64.95% <91.30%> (+0.61%)	⬆️
lib/src/processing/margins.dart	93.87% <92.85%> (+0.85%)	⬆️
lib/src/builtins/styled_element_builtin.dart	92.42% <100.00%> (+0.20%)	⬆️
lib/src/style/length.dart	100.00% <100.00%> (ø)

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