Add `standard` support out of the box

[skryukov]

I'm thinking of including exe/standard-gradual into the gem to support standard proxying.

Probably this will lead to rewriting of the glue code, that connects rubocop and rubocop-gradual.

[pboling]

This would be a cool feature! If this happens I will be able to rewrite rubocop-lts, which is a meta gem that configures rubocop-gradual, standard, and various other lint-related tools, per each specific version of Ruby. Right now it doesn't use standard's runtime, because it needs to use ruboccop-gradual instead, so it only loads standard's rules.