feat(extensions): add chrome.userScripts and chrome.offscreen

[seidnerj]

Adds two APIs that MV3-era userscript managers depend on.

chrome.userScripts: register, update, unregister, getScripts and configureWorld. Registered scripts are injected into matching frames of tracked tabs as each frame commits a navigation, honoring matches, excludeMatches, includeGlobs, excludeGlobs, allFrames and runAt.

• USER_SCRIPT world scripts run in the extension's isolated world through tabs.executeScript, so they can message the extension. MAIN world scripts run in the page.
• Injection requires a matching host permission, and file sources must resolve inside the extension.

chrome.offscreen: createDocument, closeDocument and hasDocument. Each extension may have one offscreen document, hosted in a hidden window. runtime.getContexts reports it as OFFSCREEN_DOCUMENT.

Known limitations

• Messages from user scripts arrive on runtime.onMessage/onConnect; onUserScriptMessage/onUserScriptConnect are not fired.
• configureWorld is accepted but has no effect, so CSP and messaging aren't configurable.
• Registrations are in memory, so extensions re-register on startup, as they do in Chrome.
• Scripts apply to navigations after registration.

Testing: new fixture extension and specs for both APIs; the full suite passes (68/68). Also checked manually with a real userscript manager (an MV2 build): its runtime registered through userScripts and a @grant none script ran in the page.