Add Firefox add-on

[uier]

Current code is built for chrome extension and only published to chrome web store.

For example, the manifest_version is 3, which is only supported by chrome.

Expected the extension can be used in Firefox.

[Bogay]

This might be blocked by #9

[uier]

Firefox will support manifest v3 at the end of 2022, and Chrome will deprecate v2 in 2024.

So not gonna to downgrade the manifest version, current plan of workaround is to build another package with v2.

reference: https://blog.mozilla.org/addons/2022/05/18/manifest-v3-in-firefox-recap-next-steps/