Auto merge of #9217 - KiChjang:dom-manipulation-msg, r=jdm

Redesign ScriptMsg to be more specific to DOMManipulationTaskSource

This is a large-ish PR that contains the following:
* A new directory is created under `components/script/` called `task_source`, which houses all the stuff for different task sources. Note that the ones that I have now aren't exhaustive - there are more task sources than just the generic ones.
* A `DOMManipulationTaskMsg` which eliminates some usage of `Runnable`s to fire events. Instead, they send event information to the `DOMManipulationTaskSource` and lets the `ScriptTask` handle all the event firing.
* Re-added `fn script_chan`, since I can't think of any other way to give `Trusted` values an appropriate sender.
* Rewrote step 7 of [the end](https://html.spec.whatwg.org/multipage/syntax.html#the-end) to make use of the `DOMManipulationTaskSource`

Partial #7959

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