Falling back to parent justfiles

If a recipe is not found in a justfile and the fallback setting is set, just will look for justfiles in the parent directory and up, until it reaches the root directory. just will stop after it reaches a justfile in which the fallback setting is false or unset.

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

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

set fallback
  echo foo

And the parent directory contains this justfile:

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