feat: support SPA content scripts

[Develop-KIM]

Fixes #1029

Summary

• Add an experimental spa option for isolated world content scripts.
• Register SPA content scripts at origin scope while filtering against the original match patterns at runtime.
• Restart the content script context on matching SPA navigations and abort it when navigating away.
• Add validation and tests for SPA matching and unsupported MAIN world usage.

Verification

• bun run --filter wxt check
• bun run --filter wxt test content-scripts validation spa-content-script
• git diff --check

Disclosure

This contribution was prepared with AI assistance and reviewed before submission.

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