[5.1] Rewrite treeselect in vanilla JS

[C-Lodder]

Summary of Changes

This PR removes the jQuery dependency for the treeselect Javascript.

Also uses a <div> for the dropdown heading, as opposed to a <h1>.

Testing Instructions

1. Go to edit any module in the Module Manager.
2. Navigate to the "Menu Assignment" tab
3. Select "Only on the pages selected"
4. Play around with all the treeselect options (expanding, collapsing, checking, unchecking, etc)

[ghost]

I have tested this item ✅ successfully on 29ae5d3

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[Fedik]

I have tested this item ✅ successfully on 4f775be

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[Fedik]

r2c

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[LadySolveig]

Thank you for your work @C-Lodder I tested this and as soon as an individual menu item has been manually selected/deselected, the dropdown next to it for the entire menu for "selecting/deselecting" all menu items no longer has any effect on the previously manually changed entry.
Only again after the complete page refresh.
However, this error already exists in the old version.
Could you please take a look here?

[C-Lodder]

@LadySolveig Can do, but the toolbar in the edit view is missing. I this a known bug with the recent upmerge?

[Fedik]

but the toolbar in the edit view is missing

Clear the cache for autoload_psr4.php

[C-Lodder]

@LadySolveig Good catch. Fixed now

[LadySolveig]

Thank you @C-Lodder for your quick response and the fix.
I'm very happy that you were able to fix this bug that has been around for a while!

[LadySolveig]

Thank you for your work @C-Lodder and also to the testers and support @fgsw @Fedik @dgrammatiko