Add infix operator for Scala 3

[qwbarch]

When using Scala 3 with -source:future compiler flag enabled, the compiler complains about a missing infix operator.

For example:

val authedRoutes: AuthedRoutes[User, IO] =
  AuthedRoutes.of {
    case GET -> Root / "welcome" as user => Ok(s"Welcome, ${user.name}")
  }

The compiler will complain:

Alphanumeric extractor as is not declared `infix`; it should not be used as infix operator.
The operation can be rewritten automatically to `as` under -deprecation -rewrite.
Or rewrite to prefix syntax as(...) manually.

This PR simply adds infix for the unapply method of as for Scala 3, while keeping the exact same source code for Scala 2.

[rossabaker]

👍 I didn't know about this rule. I think it's relevant back to 0.22. I'll cherry-pick it.