Add `:keys` command to list current key bindings.
This outputs the result of [`listBinds`] to a temporary file
and then pass it to `$PAGER`. A bit of care is taken to remove
the temporary file and refresh lf (just in case it's showing
`/tmp`).
I haven't tested it on Windows, since `lf` doesn't seem to work
on `wine`, but I think it should work there too.
It would be a bit more elegant to pipe the output directly to
`$PAGER` without a temporary file, but that would require larger
code changes, which doesn't seem worth it at the moment.
[`listBinds`]: https://github.com/gokcehan/lf/blob/e03143299d4596bbc9bb8a2d8a1b0931e3183410/ui.go#L940
