Fix code viewer editor when line numbers are enabled

[DamienBraillard]

When ShowLineNumbers and Editable are both set to true on the MudCodViewer, there is a shift between the caret and the shown highlighted content.

This is due to the fact that the <pre> overlay is shifted right by 3.8em and the underlying <textarea> is not. So there is effectively a shift of 3.8em between the caret and the shown highlighted content.

I encountered the issue in my project and it has also been reported in issue #628.

The fix adds a new style for the <textarea> and applies it dynamically when ShowLineNumbers is true (as it was already done for the <pre>)

The 4th doc sample example has also been updated to allow enabling line numbers when in edit mode.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.32%. Comparing base (b750a06) to head (3b2a5f0).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #638      +/-   ##
==========================================
- Coverage   65.34%   65.32%   -0.02%     
==========================================
  Files         111      111              
  Lines        8354     8361       +7     
  Branches     1474     1474              
==========================================
+ Hits         5459     5462       +3     
- Misses       2389     2393       +4     
  Partials      506      506              

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[mckaragoz]

Thanks 🥇