fix: improve API for ExtensionContext and export marker.dart

[Sub6Resources]

Fixes #1272
Fixes #1252

This simplifies the API for classes extending HtmlExtension by moving information about children to the ExtensionContext.

[codecov]

Codecov Report

Patch coverage: 72.41% and project coverage change: +0.18 🎉

Comparison is base (15cb05e) 59.85% compared to head (9fde184) 60.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1273      +/-   ##
==========================================
+ Coverage   59.85%   60.04%   +0.18%     
==========================================
  Files          37       37              
  Lines        2698     2698              
==========================================
+ Hits         1615     1620       +5     
+ Misses       1083     1078       -5     

Impacted Files	Coverage Δ
lib/src/builtins/details_element_builtin.dart	11.11% <0.00%> (ø)
lib/src/builtins/image_builtin.dart	60.39% <ø> (ø)
lib/src/builtins/ruby_builtin.dart	92.06% <ø> (ø)
lib/src/builtins/text_builtin.dart	96.15% <ø> (ø)
lib/src/extension/helpers/image_extension.dart	0.00% <0.00%> (ø)
lib/src/extension/helpers/image_tap_extension.dart	0.00% <0.00%> (ø)
lib/src/extension/helpers/matcher_extension.dart	53.84% <ø> (ø)
lib/src/extension/helpers/tag_extension.dart	66.66% <ø> (ø)
lib/src/extension/html_extension.dart	81.25% <0.00%> (ø)
lib/src/processing/lists.dart	90.90% <ø> (ø)
... and 8 more

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