[css-ui][proposal] Enhance resize capabilities and shadow DOM styling

[ydaniv]

Currently the resize property offers very limited capabilities, and UA's do not reveal any stylable shadow DOM for the author. There are many usecases today for creating resizable elements on the web (this certainly needs a research), and using resize currently only provides a solution for practically just resizing a textarea.

This was raised once in this issue: #4982, but only in relation to tables.

There are many other usecases, even without mentioning WYSIWYG editors and such. So, unlike the above mentioned issue, it would be nice to have multiple ::part's to be able to style, based on the value of resize, as well as possibly other OOTB UA behavior, e.g. changes to the start sides when those are dragged while resizing.