feat: add browser extension support

1. Add --browser-extension flag to assail command
2. Skip eval() checks for browser extensions (DevTools API usage)
3. Add new analyzer methods: new_browser_extension(), new_verbose_browser_extension()
4. Add new assail functions: analyze_browser_extension(), analyze_verbose_browser_extension()
5. Update README with browser extension guidance

Fixes false positives for Firefox/Chrome extensions using browser.devtools.inspectedWindow.eval().

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Author: hyperpolymath