Add fixer for prefer-equals-comparison rule

[anderseknert]

This is a brand new linter rule, but the PR did not include a matching fixer. Let's have it added! Working with legacy Rego sometimes mean hundreds or thousands of = usages will be flagged, so not having to fix this manually is a huge improvement for modernization efforts.

This should hopefully be an easy one to do, as the fix is simply to add another = at the reported location. Which is more or less identical to what the use-assigment-operator fix does already.

Fix should work both using regal fix from the command line, as well as via a Code Action in the language server.