[Lexer] Support combination/category tokens

[msujew]

In Langium/Xtext, to combine multiple tokens into one, one needs to write a data type parser rule:

Combination returns string: "a" | "b" | ID;

This has a bunch of implications, mainly slow performance (multiple reasons, that I won't go into here).

Inspired by chevrotain, we could collect multiple tokens into one category that acts as a "super" token that can be used in the grammar like the Combination parser rule:

token category Combination {
  // Order shouldn't matter
  "a", "b", ID
}

Hints for the implementation can be taken from chevrotain as well.