Add `get_foreground_process_name` to both Pane and the lua wrapper.
Add `foreground_process_name` and `current_working_dir` fields to
`PaneInformation`. In order for those to be dynamically fetched,
switch the lua conversion for `PaneInformation` to be a UserData
with field access methods. It's a little more verbose but allows
us to lazily compute these two new fields.
refs: https://github.com/wez/wezterm/discussions/1421
refs: https://github.com/wez/wezterm/issues/915
refs: https://github.com/wez/wezterm/issues/876