Add a command to unload the current tab

Since FireFox 140, you can unload specific tabs directly from the context menu, including the active one.

See also https://support.mozilla.org/en-US/kb/unload-tabs-reduce-memory-usage-firefox.

Please add a command to unload the current tab directly from Vimium.

I intend to bind it to U, so maybe you want to consider using this as a default binding, but it seems like a power user thing that probably not everyone will need.