Highlight `as` in givens

[nicolasstucki]

  given foo as Foo = ...
  given as Foo = ...
  given (x: X) as Foo = ...
  given [X](x: X) as Foo = ...
  given foo(x: X) as Foo = ...
  given foo[X](x: X) as Foo = ...

[nicolasstucki]

Syntax highlighting for this is particularly complex as the rule needs to identify both the given and the as. This implies that the rule would also be in charge of highlighting everything in between. Unfortunately we can have arbitrarily complex code within the parameters.

[nicolasstucki]

@odersky, when discussing all the different syntaxes of the given feature we missed this small but critical aspect of the syntax.

[MaximeKjaer]

My (WIP) implementation for this feature in Pygments is able to correctly highlight this as of this commit. The idea is to rely more on state transition than regexes. We go to a given state on (given)(\s+), and pop that state on (as)(\s+) or \{. While in that state, we highlight \(, \[ and identifiers as for a def.

[nicolasstucki]

@MaximeKjaer here are some trickier ones that you might want to check in pygments

    given foo[X <: Y { type A = 1; def f(using a: Int): 2 }](x: X = 2) as Foo = ...
    given (using x: X = "abs")(using y: Y = s"y: $x", y: Char = if true then 'a' else 2) as Foo = ...

[MaximeKjaer]

Thank you! It currently fails miserably in my implementation for Pygments, but I've added them as test cases for now!

(the pink for strings is correct, just a weird color choice of the Pygments theme; the red is error tokens)