Allow for using `yaml` file extension

[opdavies]

Allow for workspace.yaml files as well as workspace.yml.

If I create a workspace.yaml file and add a custom command to it, the command is not shown when running ws in that directory.

[opdavies]

I suppose that it would be needed to workspace.override.yaml too and any other YAML files.

I'd be happy to look into this if it's something that would be accepted.