Ensure Quill correctly detects selections and renders HTML

[philippjfr]

Some fixes for Quill rendering inside the shadow DOM.

[codecov]

Codecov Report

Merging #5511 (0e9cc57) into main (1b88afa) will decrease coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5511      +/-   ##
==========================================
- Coverage   83.60%   82.74%   -0.87%     
==========================================
  Files         276      276              
  Lines       40162    40162              
==========================================
- Hits        33579    33231     -348     
- Misses       6583     6931     +348     

Flag	Coverage Δ
ui-tests	39.67% <ø> (-1.73%)	⬇️
unitexamples-tests	72.54% <ø> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
panel/models/quill.py	90.47% <ø> (ø)

... and 20 files with indirect coverage changes

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[maximlt]

This PR was motivated by a few issues. The first one being that in some situations, re-rendering the widget would affect its content (not the model though). This can be reproduced with the following code, adding a bullet list in the editor, switching to the next view and back to the editor, where you should see the bullet lists are gone.

import panel as pn

pn.extension('texteditor')

contents = [pn.widgets.TextEditor(), pn.pane.HTML('<h1>Bob</h1>')]
contents = [pn.Column(c) for c in contents]
w_radio = pn.widgets.RadioButtonGroup(options=[0, 1])

placeholder = pn.Column(contents[0])


@pn.depends(w_radio, watch=True)
def update_view(i):
    placeholder[:] = [contents[i]]


pn.Column(w_radio, placeholder).servable()

As it currently stands, this PR fixes that issue.

The other issue I saw with the editor was that clicking on one of the tools, like B(old), the cursor would always move up to the first position in the editor. This is fixed too. However, there is still an issue with the tools. When I click on one of them like B(old), and type something, the text isn't in bold. The tool doesn't get activated (or it does but very shortly). I can set some bit of text in bold if I select it and then hit on tool.

[philippjfr]

Unfortunately I wasn't able to find a fix for the last issue yet (and no one on StackOverflow or the Quill issue tracker seems to have done so either). I'll merge for now so I can cut a dev release.