Implement firefox container tab support

[whi-tw]

Firefox can have container tabs enabled, and an extension exists that will allow routing external URLs with a specific format into these containers Open External links in a container.

This change implements logic so if configured in a handler, the outgoing URL is converted into this format when the browser is called.

This has an improvement over using a rewrite rule, in that the config can be kept DRY, and container selection logic is left to the handler. An example of such a rewrite rule implementation is here: https://github.com/johnste/finicky/issues/211#issuecomment-1047235912.

Example configuration:

handlers: [
  {
    // Some work site
    match: "https://some-site.com/*",
    browser: {
      name: "Firefox",
      container: "Work",
    },
  },
],