[Feature] Implement syntax for interface type generation

[Luna-Klatzer]

Is there an existing proposal for this?

• [X] I have searched the existing issues

This feature does not exist in the latest version

• [X] I am using the latest version

Proposal

Implement syntax for interface type generation, which allows the Lexer and Parser to map them to AST nodes.

Exact behaviour / changes you want

• [x] Add syntax for interfaces to the Lexer and Parser g4 files.
• [x] Add new interface definition AST node.

[Luna-Klatzer]

Implemented by #657.