update "supportConstant" in "CssHighlightRules"

[hrkw00]

Issue #, if available:

Description of changes:

• add flex-start and flex-end to supportConstant in src/mode/css_highlight_rules.js

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #4912 (63eb005) into master (18a459a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4912   +/-   ##
=======================================
  Coverage   85.75%   85.75%           
=======================================
  Files         539      539           
  Lines       40728    40728           
  Branches     6445     6445           
=======================================
  Hits        34928    34928           
  Misses       5800     5800           

Flag	Coverage Δ
unittests	85.75% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/mode/css_highlight_rules.js	100.00% <100.00%> (ø)

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

[andrewnester]

Thanks for your contribution!