WIP: bst fmt: Add basic functionality

Description

Adds a bst fmt command, which modifies the format of a projects element files into a canonical yaml format. This has the main benefit of automating files into the format bst track will dump them into when run.

Currently the functionality is somewhat stinted by #767, and there are a couple of other features I would like to add to this - namely an option which doesn't modify the yaml, but creates a diff, and a way for a user to define the order of top-level nodes in a .bst file, into which bst fmt sorts the nodes.

Changes proposed in this merge request:

• Add a bst fmt command which formats elements into a canonical format

This merge request, when approved, will close #485