Add off-store builds with external parser scripts loading

[kuwoyuki]

This is a draft of an off-store build variant that can load full parser scripts from remote repos at runtime. I tried to leave the store build untouched.
The motivation is this PR #2225. MV3 can't load remote code per Chrome Web Store policy. But some sites need complex decryption, login sessions, Cloudflare cookie handling, etc.
Off-store build (MV2 with unsafe-eval CSP) evals external scripts directly in the popup context so they have the full context.

• npm run build now produces 4 zips: 2 store (unchanged) + 2 off-store (Chrome MV2 + Firefox MV2, both with unsafe-eval CSP)
• Off-store build has ExternalScriptLoader.js that fetches an index.json from configured repo URLs, then fetches and evals each listed parser script
• External scripts use the same parserFactory.register() as bundled parsers so a script can be bundled or external with no changes
• If the external script registers a parser for a host that already has a bundled one, the external one wins
• Scripts cached in chrome.storage.local with optional auto-update

This will not work in MV3 (thanks Google) CSP blocks eval(). Sandboxed iframes can eval but are isolated from extension APIs, cookies, and login sessions, so that's useless for this ext.