Implement arbitrary theme variables

[MinusGix]

• Added an entry to CHANGELOG.md if this change could be valuable to users

This PR implements (sortof reimplements) a feature I've wanted for a while as it makes themes significantly easier to write & customize.
Rather than having base colors, we have general purpose variables that themes provide. These are prefixed with $.
I've also modified the default themes to have a few extra variables for common patterns in them.

• Makes it simpler to write themes, letting you have one variable you tweak to get the color right rather than copy & pasting it a half dozen times.
• Allows easier user customization of themes if all they have to change to shift the general feel of a theme to be lighter/darker/bluer/redder is a few variables rather than every location that uses the color
• Makes so if a theme defines $secondary-background and then we add a new view that uses $secondary-background, it will still be consistent with the theme. (because it falls back to default color = "$secondary-background" which is checked against the current theme first)

Possible future improvements:

• Don't sort the colors in the theme changing view
• Theme-specific alterations, to make it easy to have a basic alteration to a theme while still using it & receiving any updates
• Functions? alpha($primary-background, 0x60) and similar would allow variables to work even better, but do add some parsing complexity.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 55 lines in your changes are missing coverage. Please review.

Project coverage is 5.95%. Comparing base (43b54d0) to head (a721ff2).
Report is 37 commits behind head on master.

Files	Patch %	Lines
lapce-app/src/config/color_theme.rs	0.00%	51 Missing ⚠️
lapce-app/src/settings.rs	0.00%	3 Missing ⚠️
lapce-app/src/config.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3069      +/-   ##
=========================================
- Coverage    6.01%   5.95%   -0.06%     
=========================================
  Files         122     122              
  Lines       42703   43182     +479     
=========================================
+ Hits         2569    2572       +3     
- Misses      40134   40610     +476     

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