`-l` as a grammar locator

[zharinov]

Let -l accept more than bundled language names. It should also take a path to grammar.json, a URL, or a GitHub repo.

• check, infer, dump, and grammar only need grammar.json (no compiled parser), so they should work with any locator.
• run, trace, and source ast need a bundled parser. They must error clearly when one isn't available.
• Remote fetches are cached. CI --frozen semantics are deferred.

Acceptance

• plotnik check query.ptk -l ./grammar.json
• plotnik infer query.ptk -l https://…/grammar.json
• plotnik run … -l ./grammar.json → clear "no parser for this grammar" error