Add absolute file path command expansion value

[thomasschafer]

It's often useful to get the full path to the file in the current buffer. This PR adds a variable file_path_absolute which is expanded to the full path to the currently focused document, or the current working directory if that is not available e.g. in scratch buffers.

[the-mikedavis]

Can't this be done with realpath %{buffer_name} instead?

[thomasschafer]

Can't this be done with realpath %{buffer_name} instead?

The only difference is that this command defaults to the cwd in a scratch buffer, whereas realpath [scratch] errors, and I guess this variant is a little more ergonomic than $(realpath %{buffer_name}). Happy to close though if those aren't considered issues

[kpbaks]

realpath is a part of GNU coreutils. So having this built in would be more portable for people using helix under Windows.