Add signatures for URI::RFC2396_Parser

[ybiquitous]

This change adds signatures for the URI::RFC2396_Parser class.

See also:

• https://docs.ruby-lang.org/en/3.0.0/URI/RFC2396_Parser.html
• https://github.com/ruby/uri/blob/v0.10.1/lib/uri/rfc2396_parser.rb

[soutaro]

🪂

[ybiquitous]

Thanks for the merge! 😄