Share settings between modules

Currently, when using `import`, import happens into the parent justfile, so imports can use all the variables as well as the options that are set with `set` like `shell`, etc.

However, when another justfile is used as a module, this module entirely lives in it's own scope. While variables can atleast be exported in the parent and reexamined in the module with `shell("echo $VARIABLE")`, there is currently no way for options to be carried on.

This makes it challenging to use in many scenarios.

Context: I'm trying to build a mono-repo, with just as the task runner. Experiment here: https://github.com/prasannavl/monorepo-experiments/tree/main/just-refined

- The variables `WORKSPACE_OUT`, `WORKSPACE_ROOT` are exported. However, `shell` option needs to be set at each and every module, which might defeat the purpose in many aspects.