Add a built-in function for formatting, and sunset the Go linter

[anderseknert]

When I started hacking on Regal, I was thinking there would be a lot of things that couldn't be done using Rego alone. One obvious thing was formatting, but the plan was to add more Go linters for things that turned out to be impossible to do in Rego. I no longer think that we'll need linters written in Go, and if we find problems like that going forward, we can implement custom functions for that. Let's make one for formatting and have the opa-fmt rule use that instead. With the only Go linter gone, we can decomission that code.