feat(termwiz): add From termwiz style impls

[Valentin271]

Closes #679

Important note: this also fixes a wrong mapping between ratatui's gray and termwiz's grey. ratatui::Color::Gray now maps to termwiz::color::AnsiColor::Silver

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8f56fab) 92.1% compared to head (6b43e35) 92.3%.

Files	Patch %	Lines
src/backend/termwiz.rs	99.2%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #726     +/-   ##
=======================================
+ Coverage   92.1%   92.3%   +0.1%     
=======================================
  Files         48      48             
  Lines      14511   14769    +258     
=======================================
+ Hits       13378   13635    +257     
- Misses      1133    1134      +1     

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[joshka]

LGTM, though when merging this make sure to document the color change for the changelog.