rubocop-yard: Add support for multiple & nested Hash keys definition

[lekemula]

Description

Follow-up on #1628 (omit first 2 commits when reviewing)

Adds support for multiple and nested hash keys - example from specs:

"Hash{:key_one, :key_two => String; :key_three => Symbol; :key_four => Hash{:sub_key_one => String}}" => "a Hash with keys made of (a literal value :key_one or a literal value :key_two) and values of (Strings) and keys made of (a literal value :key_three) and values of (Symbols) and keys made of (a literal value :key_four) and values of (a Hash with keys made of (a literal value :sub_key_one) and values of (Strings))",
"Hash{:key_one => String, Number; :key_two => String}" => "a Hash with keys made of (a literal value :key_one) and values of (Strings or Numbers) and keys made of (a literal value :key_two) and values of (Strings)"

NOTE: These changes probably only impact on https://github.com/ksss/rubocop-yard and have no effect elsewhere.

PS. I think this alongside the base PR, will open up a lot of opportunities for better Hash completion/typechecking for solargraph gem.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).