Improvements for importYaml() in the js version of ksc
While working on adding `meta.imports` support to `kaitai-struct-loader`, I've noticed that it's quite tricky to track the current directory as `ksc` traverses imports. Doing so is necessary in order to construct the full path to the file on disk for loading in `importYaml()`.

Because there's no way to relate the call to `importYaml()` back to the parent `ksy` requesting the import, I'm not aware of a way I can always correctly resolve the full path on disk for imports, particularly when dealing with sibling imports from varied paths.

Per @koczkatamas's suggestion the other day, perhaps `importYaml()` could be simplified to something like: `importYaml(path)`, where `path` either:

* starts with a `/` to denote an absolute path
* or is a path relative to the _initial_ `ksy` being passed into `compile()`

This would likely require passing in the initial `ksy` path to `compile()` and adding some path handling logic to `ksc`.

I realize this might not be ideal. An alternative might be to change how `compile()` works, such that `importYaml()` can tie the import back to the `ksy` requesting the import. Not quite sure what that might look like.