[whatwg/dom] Side effects due to tree insertion or removal (script, iframe) (#808)

Elements that trigger a script-executing side effect due to insertion or removal need to do so using some kind of queue if we ever want to remove mutation events fully (see #305). In particular, I think it's important that tree mutation succeeds and that any script is executed afterwards (but before custom element reactions, see https://github.com/web-platform-tests/wpt/pull/20660#event-2862047798).

I'd like to use this thread for discussion, since currently it's spread among various places (https://github.com/whatwg/dom/issues/575, https://github.com/whatwg/dom/pull/732, https://github.com/whatwg/html/pull/4354, https://github.com/whatwg/html/issues/4611, https://github.com/whatwg/html/issues/4965).

Chrome and Firefox have this model for `script` insertion. (Though there are differences, if two scripts are inserted and the first removes the second, Firefox will execute the second, Chrome will not. They also react differently to changes to children of the script element, with Chrome only caring about insertions there.)

Chrome has this model for `iframe` insertion as well, but not removal. Firefox only has it for removal (insertion cannot trigger script in Firefox as it does not have a synchronous `load` event).

Firefox has this model for `style` insertion, but since `style` cannot trigger script I don't think we should follow that.

@nox @bzbarsky @tkent-google @rniwa thoughts? Do we all share the overall goal of executing script at a "safe" point? How would you like to see this broken down further?

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