feat: add option to keep running when browser is closed

Closes #6 (closed)

TODO:

• Make different builds for Gecko and Chromium, ensure this doesn't affect the <iframe> version.
• Make sure it actually makes sense to merge (!30 (comment 2829084))

Can be done later TODO:

• Localize the string (can someone tell me how?)
• Maybe adjust styles if the string gets long in some locales. They're OK-ish as is, but maybe.
• Write tests? Although the test script would need to be adjusted as there are no tests for static/popup.js and init-webext.js right now.
• When "enabled" is false and run-in-background is true, don't keep running. Maybe make it a separate MR.
• Make sure the changes to make.js don't conflict with README.md. I have updated it, but not sure if NPM / webpage workflows are not broken.