Updated Encoding, its Errors, and its tests

[sampersand]

This updates the signatures and tests of Encoding and its errors (Encoding::CompatibilityError, Encoding::ConverterNotFoundError, Encoding::InvalidByteSequenceError, and Encoding::UndefinedConversionError).

Notably, this does not update Encoding::Converter, which will be a separate PR.

More specifically, this PR updates the following signatures:

• Encoding._load: Added, but without documentation (as it doesn't have any).
• Encoding.locale_charmap: Added
• Encoding.default_external=, and Encoding.default_internal=: Updated to match _ToStr. This signature could be improved when Allow Type Variable constraints to be arbitrary types #1471 merges.
• Encoding.find: Use encoding instead of String | Encoding.
• Encoding.aliases, Encoding.list, Encoding.name_list, Encoding#names: Remove leading :: from signatures
• Encoding#replicate: REMOVED ENTIRELY. See Remove Encoding#replicate ruby#7079 for details.
• Encoding#to_s: Now an alias of Encoding.name
• All the Encodings and errors are now underneath Encoding instead of at toplevel.

It makes the other following changes:

• Moves Encoding_InvalidByteSequenceError_test.rb into Encoding_test.rb
• Moves Encoding_UndefinedConversionError_test.rb.rb into Encoding_test.rb
• Creates Encoding_CompatibilityErrorInstanceTest and Encoding_ConverterNotFoundErrorInstanceTest test classes.
• Updates Encoding_test.rb to use the modern testing suite, testing the new new version of signatures.

[soutaro]

Looks good! Fixing the CI errors.