Provide support for Scala 3

[alexandru]

The name of the language Scala 3 is the next iteration of the Scala language, already released and stable — Rouge supports Scala 2 syntax, but Scala 3 also introduces some new keywords (e.g., enum, end), and it adds the option to use significant indentation.

Implementation in other libraries Pygments has already implemented support for Scala 3, inspired by the Scala lexer defined for VS Code, see:

• PR #1694 from Mar 1, 2021;
• highlighting sample;

I'm a Scala developer, I have a Scala personal blog built with Jekyll and configured with Rouge, I'd really appreciate it.

Thanks for all your wonderful work ❤️

[hearnadam]

I plan to work on this later in the week.