Add syntax highlighting for Flix

[sockmaster27]

Description of changes:
Adds syntax highlighting for the Flix programming language:
https://flix.dev/
https://github.com/flix/flix

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (3086776) 87.46% compared to head (7b74feb) 87.46%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5356   +/-   ##
=======================================
  Coverage   87.46%   87.46%           
=======================================
  Files         577      579    +2     
  Lines       45858    45880   +22     
  Branches     6954     6954           
=======================================
+ Hits        40109    40131   +22     
  Misses       5749     5749           

Flag	Coverage Δ
unittests	87.46% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
src/mode/flix.js	100.00% <100.00%> (ø)
src/mode/flix_highlight_rules.js	100.00% <100.00%> (ø)

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[InspiredGuy]

Thank you for the contribution!