fix: Cleaned up whitespace processing and added whitespace tests

[Sub6Resources]

Fixes #1146 and fixes #1251

[codecov]

Codecov Report

Patch coverage: 87.27% and project coverage change: +0.42 🎉

Comparison is base (fe896de) 59.31% compared to head (46fb455) 59.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1267      +/-   ##
==========================================
+ Coverage   59.31%   59.73%   +0.42%     
==========================================
  Files          37       37              
  Lines        2652     2690      +38     
==========================================
+ Hits         1573     1607      +34     
- Misses       1079     1083       +4     

Impacted Files	Coverage Δ
lib/src/builtins/styled_element_builtin.dart	92.51% <37.50%> (-4.30%)	⬇️
lib/src/processing/whitespace.dart	85.47% <95.12%> (+0.58%)	⬆️
lib/src/builtins/text_builtin.dart	96.15% <100.00%> (+0.32%)	⬆️
lib/src/tree/replaced_element.dart	63.15% <100.00%> (+4.33%)	⬆️

... and 3 files with indirect coverage changes

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