Rename module script to JavaScript module script

This patch creates a common concept of a "module script", unifying
JavaScript module scripts with JSON module scripts (and future concepts
like style sheet module scripts, WebAssembly module scripts, etc.)
The name change includes the following fixes:
- Clarify that <link rel=modulepreload> works for JSON module scripts.
- Check for Cyclic Module Records in fetching module descendants
  and finding the first parse error.
- Initialize the settings object and error to rethrow in JSON module
  scripts, both of which are used by algorithms in HTML.
- Add various asserts to make it clear why we can use things when we do.

Module scripts may be renamed to simply "modules" in a future patch.