Allow using regal fix as a formatter

[anderseknert]

It would be nice to be able to set the equivalent LSP command for regal fix as the formatter for Rego, which would make it possible to have "format on save" run regal fix instead of opa fmt. Since regal fix is 100% compatible with opa fmt, this would simply be fmt with some nice additions, and more to come if we start looking into ordering of imports, and so on.

Looking into if it's possible to provide this via the LSP initialization options from the client would be a good first step.