add internal gutter cursor marker feature for cloud-editor themes

[nightwing]

themes that want to show border around active line may want to highlight only part of the wrapped line.

Description of changes:

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

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

❌ Patch coverage is 59.25926% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.85%. Comparing base (900cffc) to head (517cdbc).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/gutter.js	45.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5873      +/-   ##
==========================================
- Coverage   87.88%   87.85%   -0.04%     
==========================================
  Files         622      622              
  Lines       46412    46441      +29     
  Branches     7606     7613       +7     
==========================================
+ Hits        40790    40799       +9     
- Misses       5622     5642      +20     

Flag	Coverage Δ
unittests	87.85% <59.25%> (-0.04%)	⬇️

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