add z-index: 1 to ace_placeholder

[InspiredGuy]

Issue #, if available:
#5361

Description of changes:
Add z-index: 1 to ace_placeholder to make it appear on top of the active line indicator.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 86.89%. Comparing base (b7495e1) to head (c607b22).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5612   +/-   ##
=======================================
  Coverage   86.89%   86.89%           
=======================================
  Files         594      594           
  Lines       43143    43143           
  Branches     7150     7150           
=======================================
  Hits        37489    37489           
  Misses       5654     5654           

Flag	Coverage Δ
unittests	86.89% <ø> (ø)

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