#58 test: add a testbench page

Summary by cubic

Adds a browser testbench demo page that runs Ghostbird against mocked Thunderbird APIs, so the extension can be tried without Thunderbird.

• Ghostbird now takes body and selection as constructor parameters instead of reading them from document.
• The testbench bundle is built into dist/testbench/main.js only in development builds.
• Documents how to run the demo page and updates the linter config to cover HTML files.