Falling back to parent justfiles

If a recipe is not found, just will look for justfiles in the parent directory and up, until it reaches the root directory.

This feature is currently unstable, and so must be enabled with the --unstable flag.

As an example, suppose the current directory contains this justfile:

foo:
  echo foo

And the parent directory contains this justfile:

bar:
  echo bar
$ just --unstable bar
Trying ../justfile
echo bar
bar