Add BlockNode#{first,last}_argument helpers

[sambostock]

These methods allow more consistent use of first_argument and last_argument across more node types.

The motivation for this PR is to allow for an InternalAffairs/NodeFirstArgument cop to enforce node.arguments.first and node.arguments[0] be replaced with node.first_argument. For this to be possible, we need to extend first_argument to other node types, since the cop can't statically know what the node type is to determine if the method exists or not.

[sambostock]

This CI failure seems unrelated:

spec/rubocop/ast/rubocop_compatibility_spec.rb:3:1: C: RSpec/FilePath: Spec path should end with rubocop/ast/rubo_cop_compatibility*_spec.rb.
RSpec.describe RuboCop::AST::RuboCopCompatibility do
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[koic]

This CI failure seems unrelated:

JFYI, that offense has been suppressed in the master branch.

[sambostock]

Thanks, @koic. I've rebased, and also extracted the typo fix to #271.

[marcandre]

Nice, thank you

[marcandre]

Released as 1.30.0