Refactor ScriptLoader per-spec

In order to

- Reflect HTML spec updates, mainly
  https://github.com/whatwg/html/pull/7876, and
- Make the intervention logic clearer.

Namely, this CL largely refactors the final part of
`PrepareScript()` (corresponding to Steps 31 and 32),
by changing the code structure according to html#7876
and restructuring the logic into three steps:

1. Determine per-spec scheduling type in
   `GetScriptSchedulingTypePerSpec()`.
2. Modify the scheduling type due to interventions, and
3. Actually schedule scripts based on the scheduling type
   and interact with ScriptRunner and parsers.

Other parts are more straightforward and somehow mechanical:

- Renaming ScriptLoader::`non_blocking_` to `force_async_`.
- Updating spec comments (using
  https://docs.google.com/document/d/1vmnpbDvMR-ph0v88lC810xsE6kjWVsQP351_l65pxLs/edit#heading=h.2obuaw96wltr).
- Other very minor refactoring.

This CL shouldn't change the behavior.

Bug: 1344772, 1340837, 1339112
Change-Id: I788f682531fa0d417338562a4f0420f4c8b49e72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3795585
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1031114}