fix: syntax highlighting lost when toggling Editable on MudCodeViewer

[DamienBraillard]

When the Editable parameter changes, syntax highlighting is lost
until the text is modified.

This happens because no parameter scope is registered for Editable,
so ParameterChanged is never invoked on change. As a result, the
MudCode.highlight JS function isn't called after the DOM updates,
leaving the code unhighlighted.

I ran into this in my project; it's also reported in #628.

The fix registers Editable in the parameter scope, the same way as
Code, ShowLineNumbers, etc. This ensures ParameterChanged fires
when Editable changes, triggering MudCode.highlight to re-highlight
the code after the DOM update.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.39%. Comparing base (6051e0e) to head (8258db1).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #642      +/-   ##
==========================================
+ Coverage   65.38%   65.39%   +0.01%     
==========================================
  Files         111      111              
  Lines        8381     8384       +3     
  Branches     1480     1480              
==========================================
+ Hits         5480     5483       +3     
  Misses       2394     2394              
  Partials      507      507              

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