Brings as into line with the other operators wrt Focus

[kenbot]

• Thanks to Scala 3 RC1, adds a From parameter to the Focus as keyword, constrained by a type bounds. This is now the same signature as the external as operator:

extension [From] (from: From)
      def as[CastTo <: From]: CastTo

• Adds a case to the FocusKeyword pattern matcher to correctly interpret an extension method with both inferred and direct type parameter lists.
• Renamed GenPrism to AsPrism. It is no longer analogous to the deprecated GenXXX features, and should not forward to Focus. It is an implementation detail for the new and supported as operator.
• Changed the AsGenerator implementation to directly use AsPrism, rather than have its own operation. This is now the same as the other features, which all generate code that delegates to the external operator. We are using the implementation class AsPrism rather than the direct as operator because the external one is only accessible as an optic extension method. The way Focus works, each optic is generated on its own and composed together later, so we can't call an extension method on the last thing generated.
• Removed the CastTo <: To constraint in the AsPrism implementation, because proving the relationship was hard inside the Focus macro. It is sufficient to enforce the constraint in the public interface.
• Added some tests to confirm that the Focus and external as operators commute with respect to each other, just like the other operators

[julien-truffaut]

Nice

[julien-truffaut]

Great!