Add penalty on case mismatch to completion filter

[jonpyt]

Issue #, if available: N/A

I read through the code of the filterCompletions method and saw a TODO comment asking to implement a penalty on case mismatch. Not entirely sure what would be a good value for the penalty, but I went with 3 for each character and it works well enough.

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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 86.92%. Comparing base (0192880) to head (3053e0d).
Report is 125 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #5626 +/- ## ======================================= Coverage 86.91% 86.92% ======================================= Files 594 594 Lines 43222 43246 +24 Branches 7155 7156 +1 ======================================= + Hits 37568 37593 +25  + Misses 5654 5653 -1 

Flag	Coverage Δ
unittests	86.92% <100.00%> (+<0.01%)	⬆️

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