Add custom cop to disallow "let!"

[klappradla]

Add a custom cop to check for the usage of let! in RSpec test setups. Background: we prefer to use the lazy-evaluated let and call it in an explicit before hook.

RSpec's let! does the same under the hood, but it's visually less obvious and tends to be overused by people.

Example:

# Bad
let!(:thing) { create(:thing) }

it 'does things' do
  expect(thing.do).to eq(1)
end

# Good
let(:thing) { create(:thing) }

before { thing }

it 'does things' do
  expect(thing.do).to eq(1)
end

For adding a custom cop, see:

• https://docs.rubocop.org/rubocop/extensions.html
• https://docs.rubocop.org/rubocop/development.html
• take LetSetup as inspiration