feat: dark mode improvement

[hyj1204]

1. Add popupMenuFGColor for all the text and icon color used on the item's pop up menu
2. Add popupMenuHoverColor for all the hover color used on the item's pop up menu
3. Add a new clear icon to adapt to both light and dark mode
   
   

These changes mainly impact the following UIs

1. Color pickers

2. Link pop up menu

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #90 (00f0f8c) into main (0f303cb) will decrease coverage by 0.01%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   86.79%   86.79%   -0.01%     
==========================================
  Files         124      124              
  Lines        7322     7337      +15     
==========================================
+ Hits         6355     6368      +13     
- Misses        967      969       +2     

Impacted Files	Coverage Δ
lib/src/render/color_menu/color_picker.dart	96.12% <85.71%> (-1.52%)	⬇️
lib/src/render/link_menu/link_menu.dart	98.48% <92.85%> (-1.52%)	⬇️
lib/src/render/style/editor_style.dart	100.00% <100.00%> (ø)
lib/src/render/toolbar/toolbar_item.dart	94.07% <100.00%> (+0.30%)	⬆️

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