Use ResizeObserver to call L.Popup's update()

[IvanSanchez]

Feature request, very similar to #8222 , but for popups instead of for the whole map.

The idea consists in attaching a ResizeObserver to the DOM element with the popup's (or the tooltip's) contents. Whenever the contents change in such a way that the container box is resized, call the update() method.

This needs a bit of research, in regards of where this functionality should go. Ideally, this would be implemented in DivOverlay so it works on both popups and tooltips.

Kinda a follow-up to #8217 . I expect there will be more than one framework, or web component, or whatever, capable of modifying the contents of a popup/tooltip without explicitly calling update().

Exactly as in #8222, this won't work on any version of IE, nor Edge < 79 (that includes all the Edge versions previous to the switch to chrome's blink engine).