Updated the CSS Syntax Highlighter

[TimChinye]

Updated the mode-css.js file to the latest CSS specifications, using MDN provided information and Ace Linters (As of May 29).

Issue #, if available: #5559

Description of changes: Updated the mode-css.js file to latest CSS specifications, using MDN provided information and Ace Linters.

Additional Info:

• Ace Linters supports Non-standard and Deprecated code, but not Experimental - this CSS Mode complies with that.
• khz and hz units are no longer supported, as they're technically experimental (as in - they have not use "yet"), gd unit has been removed as it's seemingly a myth. Was mentioned in a single version of a CSS specification with zero information to go along with it, and removed within a year.
• Still does not take into consideration the new CSS Nesting.

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

Original Issue: ajaxorg/ace-builds#264 (comment)

Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (6dfb903) to head (a48a550).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5676   +/-   ##
=======================================
  Coverage   87.01%   87.01%           
=======================================
  Files         598      598           
  Lines       43680    43680           
  Branches     7204     7204           
=======================================
  Hits        38009    38009           
  Misses       5671     5671           

Flag	Coverage Δ
unittests	87.01% <100.00%> (ø)

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