Implement tracked members

[KacperFKorban]

closes #21754

Allow for the tracked modifier to be used for val members of classes and traits. tracked members and members inheriting from tracked force the type of the member (or it's overriding member) to be as exact as
possible. More precisely, it will will assign the tracked member the infered type of the rhs. For instance, consider the following definition:

trait F:
  tracked val a: Int
  tracked val b: Int

class N extends F:
  val a = 22 // a.type =:= 22
  val b: Int = 22 // b.type =:= Int
  tracked val c = 22 // c.type =:= 22

Here, the tracked modifier ensures that the type of a in N is 22 and not Int. But the type of b is N is Int since it's explicitly declared as Int. tracked members can also be immediately initialized, as in the case of c.

[odersky]

This PR needs an update to the spec. I believe we should update docs/_docs/reference/experimenta/modularity.md and also docs/_docs/internals/syntax.md. With a spec we can better evaluate whether the implementation is correct.

internals/syntax.md describes the syntax supported by the parser, including all language imports.

[odersky]

LGTM. The changes are very clean.