Does not work with MPF 0.57

[ericselk2018]

This file:

https://github.com/missionpinball/mpf-ls/blob/dev/mpfls/workspace.py

References this file:

https://github.com/missionpinball/mpf/blob/0.56.x/mpf/file_interfaces/yaml_roundtrip.py

Which was removed as of 0.57.

Also, the readme install should probably have "pip install mpf-language-server" as the example install command, since all of the MPF docs have been switched back to using pip and not pipx, and also running LS in a venv comes with other issues like vscode not being able to find it, so probably best to install globally (at least for novice users following default instructions).