feat(layouts): global cwd

[imsnif]

This change does two things:

1. It allows defining a cwd for non-command "normal" terminal panes too. So that we can define panes that start at a certain folder.
2. It allows defining a global cwd per layout. This cwd will be given to all panes in the layout. If they have a relative cwd, it will be appended to this cwd, if this have an absolute cwd it will override this cwd.
3. When opening a new tab from the cli with a layout using the zellij action new-tab --layout /path/to/my/layout.kdl, we can now add a --cwd flag that will be used just like the global cwd in 2, and override it if present. This should allow much more flexibility when creating custom layouts.

Thanks @har7an for suggesting this feature!

EDIT: updated the documentation in #1759 - which is where we currently keep our docs for this until release time.