Allow the editor in read-only to tighten to the actual necessary width

[amantoux]

This PR allows editor to shrink to max content size in read-only

To acheive this, one can use a new paramter textWidthBasis

FleatherEditor(
   controller: controller,
   scrollable: false,
   readOnly: true,
   enableInteractiveSelection: false,
   textWidthBasis: TextWidthBasis.longestLine,
);

How it renders

Before

After

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.93%. Comparing base (5b3a1d7) to head (46db52b).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/fleather/lib/src/rendering/editor.dart	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   88.71%   88.93%   +0.22%     
==========================================
  Files          63       63              
  Lines       10857    10876      +19     
==========================================
+ Hits         9632     9673      +41     
+ Misses       1225     1203      -22     

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[Amir-P]

We really needed it! Thanks! I just have two small comments. PTAL.

[amantoux]

@Amir-P Fixed accordingly, can you please take a look?

[Amir-P]

LGTM. Thanks!