Load a Prism grammar only when a blob names it

Load a Prism grammar only when a blob names it

The worker imported prism-languages.min.js — every grammar Prism ships — on
every code page. That is 546 KB to evaluate before the first token, and a page
showing one Go file paid it for all 293 components.

Each component is now vendored as its own file, and prism-manifest.js maps a
language name to the files that define it in dependency order. The worker
starts with Prism's core and the manifest and imports a language's files the
first time a job names it, remembering what it has run so a file is never
imported twice. A Go blob loads 30 KB instead of 564 KB; a .tsx blob, the
widest common entry at 8 files, loads 49 KB.

The manifest also carries the components that register no language of their
own but change one: css-extras, js-extras, js-templates, php-extras, xml-doc.
Each is placed after the grammar it rewrites and included in the entries that
inherit it, so a language that extends javascript now gets js-extras' arrow
and maybe-class-name tokens too. The old bundle's order handed those to some
inheriting languages and not others; nothing loses a token.

scripts/vendor-prism.py writes the grammars and the manifest, and --check
compares them byte for byte with cdnjs. TestPrismLanguagesAreVendored,
TestPrismGrammarsMatchTheManifest and TestPrismWorkerLoadsGrammarsOnDemand
hold the server's extension table, the vendored set and the worker's lazy
import together.