Add a Command to Run 'scalafix-run'

[LaurenceWarne]

Hi, this PR adds a new command lsp-metals-run-scalafix which runs the scalafix-run command (documentation: https://scalameta.org/metals/docs/integrations/new-editor/#run-all-scalafix-rules).

~I've also added a utility function for adding this command to before-save-hook, and added this to the example setup~. I hope this is helpful, thanks :slightly_smiling_face: