Move worklets into the HTML Standard

Closes w3c/css-houdini-drafts#1000.

This provides a baseline by porting over all the existing text from
https://drafts.css-houdini.org/worklets/, modernizing and restructuring
it along the way. It does not yet fix many of the open logged issues
(although it does fix some; see below).

Notable changes from that document:

* Rearranged sections to better match workers, and my sense of flow.

* Moved worklet script fetching to be siblings with all the other script
  fetching algorithms.

* Improved clarity and guidance on what specifications that define
  worklets should do, including fleshing out the fake worklet example.

* Changed "create a WorkletGlobalScope", which took one set of
  arguments, to "create a worklet global scope", which just takes a
  Worklet instance. This appears to match better how the algorithm is
  used, e.g. in
  https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image step
  10.

* Updated "report an error" to bail out for non-EventTarget globals,
  like WorkletGlobalScope. Closes whatwg#2611.

* Updated worklets to only be exposed in secure contexts. Closes
  w3c/css-houdini-drafts#505.

* Makes the lifetime of creating and terminating WorkletGlobalScopes
  more explicit. Closes
  w3c/css-houdini-drafts#224. Closes
  w3c/css-houdini-drafts#389.

* Explicitly start and stop the event loop for a given
  WorkletGlobalScope upon creation/termination. Closes
  w3c/css-houdini-drafts#843.
  Closes w3c/css-houdini-drafts#318 for real.

* Fixes creation of new worklet global scopes to only run the top-level
  module scripts added via addModule(), which will automatically run
  their dependencies. Previously it would run all module scripts loaded
  into the worklet, so dependencies would be run in the order they were
  fetched, not as part of the top-down module evaluation process. Closes
  w3c/css-houdini-drafts#264.