Support simple type inference for let methods

[lekemula]

Right now let completion does not have any time inference. It would be nice to have return types assigned to let-like methods, for example:

RSpec.describe SomeClass do
  let(:my_variable) { SomeClass.new } # => # @return [SomeClass]
  let(:my_variable) { described_class.new } # => # @return [SomeClass]
end

relates to: castwide/solargraph#325