Fix #10050: Use lowercase pattern variable syntax for quoted patterns

[nicolasstucki]

Use lower cases for quoted type patterns

case '{ $x: T } => '{ val y: T = .. .} // T from outside
case '{ $x: t } => '{ val y: t = .. .} // t form within the quotes
case '{ type t; $x: Map[`t`, `t`] } => '{ val y: t = .. .} // t form within the quotes
case '{ $x: `t` } => '{ val y: t = .. .} // t from outside

Based on #10050 (comment)

[smarter]

case '{ type t; $x: Map[`t`, `t`] } => '{ val y: t = .. .} // t form within the quotes

Would the following be possible?

case '{ $x: Map[t, t] } => '{ val y: t = .. .} // t form within the quotes

Semantically this makes sense I think: we accept a type in all these positions, it's just that it has to be the same one. Normal pattern syntax does not currently allow referring to the same pattern variable multiple times in a pattern, but I recall @LPTK had some ideas on how this could be useful (e.g., when using alternatives).

[nicolasstucki]

@smarter in theory it is possible. It is the next step that I want to try. The change needed to support this is unrelated to the change in syntax and I would add it in another PR.

[LPTK]

It would be great if the change made to allow repeating type pattern variables would be extended to all patterns, not just quoted patterns.

As for supporting term pattern variables in pattern alternatives, that's a whole other beast!

[nicolasstucki]

Rebased and resolved conflicts