Change curly braces insertion behavior for Markdown

[mkslanc]

Issue #, if available: #4199

Description of changes:
Now curly braces insertion would act the same as [] and () for Markdown

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[codecov]

Codecov Report

Base: 86.08% // Head: 86.08% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9716e60) compared to base (0c49167).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4994   +/-   ##
=======================================
  Coverage   86.08%   86.08%           
=======================================
  Files         544      546    +2     
  Lines       41511    41556   +45     
  Branches     6547     6551    +4     
=======================================
+ Hits        35735    35774   +39     
- Misses       5776     5782    +6     

Flag	Coverage Δ
unittests	86.08% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/mode/markdown.js	67.85% <100.00%> (-2.15%)	⬇️
src/virtual_renderer.js	79.37% <0.00%> (-0.46%)	⬇️
src/lib/keys.js	100.00% <0.00%> (ø)
src/mode/jexl_highlight_rules.js	100.00% <0.00%> (ø)
src/mode/jexl.js	100.00% <0.00%> (ø)
src/lib/dom.js	83.59% <0.00%> (+0.08%)	⬆️
src/ace_test.js	98.86% <0.00%> (+0.16%)	⬆️

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