Add rbs types and corresponding lint check

[gonzedge]

First:

• Add rbs gem and all rbs types in sig/ directory
• Add steep gem for easier rbs type checking, with basic configuration only checking lib for now
• Add rubyzip types as they are not defined

Then:

• Add || raise for places where we expect to always have a value, but that rbs/steep are not smart enough to autodetect
• Add EMPTY_ENUMERATOR to be returned when there is no iteration to perform
• Add _Nilable interface to allow for nil? check on generic type for ProviderCollection#contains?
• Expand ProviderCollection#resolve to key? check to prevent rbs/steep from complaining about Cannot have body method
• Add children_match_prefix partial_word_chars? word_chars? closest_node private abstract methods to Nodes::Node, so that both Nodes::Raw and Nodes::Compressed have it without duplication
• Make yardoc match rbs types

Finally:

• Rename char_symbols to reversed_char_symbols for clarity
• Rename Nodes::Raw#add's param chars to reversed_chars for clarity
• Add lint-rbs-steep action
• Rename lint => lint-rubocop
• Bump lint and coverage ruby versions to 3.3.6

[codeclimate]

Code Climate has analyzed commit 8001db0 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (95% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

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