Added scopes for many kernel and special methods in tree-sitter parser

[gsmetal]

Description of the Change

Added scopes for many kernel and special methods in tree-sitter parser. For example:

• visibility methods (private, protected ...)
• new method
• Gemfile-specific methods
• lambda notation (->(a) { })
• attr_accessors
• some predefined constants (ENV, __FILE__)

Alternate Designs

None.

Benefits

More accurate highlighting.

Possible Drawbacks

None.

Applicable Issues

#252

Disclaimer

1. I didn't understand how to "extend" grammars to make Gemfile's grammar separate and just extend ruby grammar with some specials. Is it possible?
2. I haven't found any explicit information about available scopes and logic for mapping them to each language-specific situation. So I've done my best effort.

[chbk]

As a heads up, except for the gemfile specific methods, #291 provides scopes for everything in this PR.