Revamp script element introduction and conformance

Notably, fixes whatwg#10070 by introducing a table clarifying which attributes are valid for which types of scripts.

Includes various editorial improvements and modernizations, e.g. settling on a uniform order for listing the attributes, breaking up different topics with `<hr>`s, and being clearer about external vs. inline scripts and initial fetches vs. imported module fetches.

Removes some conformance requirements on the contents of CSS and JSON module scripts, since external script elements can only point to JavaScript module scripts.

Normative changes are only to document conformance requirements, filling in gaps where they previously failed to disallow certain attribute combinations.

Author: domenic