Add Nord theme (https://www.nordtheme.com/)

[ghost]

Issue #, if available: #4202

Description of changes: Make the popular Nord theme available to Ace.

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

[codecov]

Codecov Report

Merging #4203 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4203      +/-   ##
==========================================
- Coverage   70.77%   70.77%   -0.01%     
==========================================
  Files         527      527              
  Lines       51464    51464              
  Branches     9656     9656              
==========================================
- Hits        36424    36423       -1     
- Misses      15040    15041       +1

Impacted Files	Coverage Δ
lib/ace/range.js	84.82% <0%> (-0.69%)	⬇️

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[nightwing]

There are two issues with the theme: selection and active line colors are same, so it is not possible to see single line selections, comments are not visible over selection.

[ghost]

Fixed. Should be okay to merge now

EDIT: Seems like comments are still not visible over a selection. :/

[nightwing]

I found we had a workaround for themes that set same color to selection and highlight, could you please check if after my changes theme looks correct, or if we should make some other changes?

[ghost]

Made gutter color match Sublime, LGTM

[nightwing]

Thanks!