Add image link parsing to the Markdown link parsing RegEx

[RickStrahl]

Image link highlighting in mode-markdown seems to not highlight the leading ! character. In some cases this looks OK in others maybe not so much. I think it's probably better to have the entire image formatting use the same colors.

Added in explicit image link handling.

This is what it looks like before:

and here it is after the change:

The latter seems more consistent.

To be honest this is minor but several of my users have mentioned this (here's one) and looking at the two renderings side by side I do think that the latter looks more consistent.

Note: Resubmitted as a new PR due a face palm on my end on the previous PR #3667. Sorry for the noise.

[codecov]

Codecov Report

Merging #3669 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3669      +/-   ##
==========================================
- Coverage   66.54%   66.54%   -0.01%     
==========================================
  Files         482      482              
  Lines       47721    47721              
  Branches     9082     9082              
==========================================
- Hits        31755    31754       -1     
- Misses      15966    15967       +1

Impacted Files	Coverage Δ
lib/ace/mode/markdown_highlight_rules.js	43.18% <ø> (ø)	⬆️
lib/ace/virtual_renderer.js	68.5% <0%> (-0.12%)	⬇️

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