Introduce `EmptyLineAfterFinalLetItBe` cop

Description of the proposal

This MR introduces a new cop to match the behavior with the existing EmptyLineAfterFinalLet cop.

# bad
let_it_be(:foo) { bar }
let_it_be(:something) { other }
it { does_something }

# good
let_it_be(:foo) { bar }
let_it_be(:something) { other }

it { does_something }

Check-list

• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend)
• [-] If there is a choice to make between two potential styles, set up an emoji vote in the MR:
  • CHOICE_A: 🅰
  • CHOICE_B: 🅱
  • Vote yourself for both choices so that people know these are the choices
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus)
• [-] (If applicable) One style is getting a majority of vote (compared to the other choice)
• [-] (If applicable) Update the MR with the chosen style
• [-] Follow the review process as usual

/cc @gitlab-org/maintainers/rails-backend