feat: make remove widget work for slivers

[tenhobi]

Adds option to use Remove this widget also on Sliver widgets (with sliver and slivers argument).
Fixes #56637

Also adds ability to remove widget without child/children/sliver/slivers/builder arguments, if the widget is in a list.
Fixes #56390

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[JulianHock]

Great job @tenhobi ! Do you have an idea how long it will take to have this in "stable"?

[tenhobi]

@JulianHock no idea. I suppose next Dart release should have this included? But I am really not sure how these things work tbo. 😁