Add RFC2396_PARSER to URI module

[sled]

The URI module defines the constant URI::RFC2396_PARSER here but it was missing in the RBS declaration, so I got the following warning:

URI::RFC2396_PARSER.escape("https://example.org")

something.rb:32:29: [warning] Cannot find the declaration of constant: `RFC2396_PARSER`
│ Diagnostic ID: Ruby::UnknownConstant
│
└         URI::RFC2396_PARSER.escape("https://example.org")

[sled]

Rubocop causes a segmentation fault, I guess this is unrelated to the change.

[ksss]

Thanks!