Allow import x.{*, given} under -Xsource:3

Imagine a Scala 3 library containing:
```
object A {
  val a: Int = 1
  given b: Int = 2
}
```
To import all members of `A` from Scala 2, we write `import A.*`,
but to do the same from Scala 3, we need to write `import A.{*, given}`
instead. This complicates cross-compilation for projects which depend on
Scala 3 libraries (unless these libraries exclusively use `implicit`
which is not something we want to encourage).

This commit remedies this by allowing `import x.{*, given}` (and `import
x.{given, *}`), this is easy to do since we can just pretend the user
wrote `import x.*` which will give us both regular and given members in
Scala 2 code and therefore match the semantics of Scala 3.

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2693,6 +2693,30 @@ self =>
       })
     }
 
+    /** Treat an import of `*, given` or `given, *` as if it was an import of `*`
+     *  since the former in Scala 3 has the same semantics as the latter in Scala 2.
+     */
+    def scala3FullSelector(): ImportSelector /* | Null */ = {
+      if (in.token != IDENTIFIER)
+        return null
+      val other =
+        if (in.name == raw.STAR)
+          nme.`given`
+        else if (in.name == nme.`given`)
+          raw.STAR
+        else
+          return null
+
+      if (lookingAhead(in.token == COMMA && {
+          in.nextToken(); in.token == IDENTIFIER && in.name == other })) {
+        val start = in.offset
+        in.nextToken() // `*` or `given`
+        in.nextToken() // `,`
+        in.nextToken() // `given` or `*`
+        ImportSelector(nme.WILDCARD, start, null, -1)
+      } else null
+    }
+
     /** {{{
      *  ImportSelectors ::= `{` {ImportSelector `,`} (ImportSelector | `_`) `}`
      *  }}}
@@ -2712,6 +2736,11 @@ self =>
      *  }}}
      */
     def importSelector(): ImportSelector = {
+      if (settings.isScala3) {
+        val fullSelector = scala3FullSelector()
+        if (fullSelector != null) return fullSelector
+      }
+
       val start        = in.offset
       val bbq          = in.token == BACKQUOTED_IDENT
       val name         = wildcardOrIdent()

test/files/pos/import-future.scala

@@ -26,7 +26,7 @@ class C {
 
 object starring {
 
-  import scala.concurrent.*, duration.{Duration as D, *}, ExecutionContext.Implicits.*
+  import scala.concurrent.{*, given}, duration.{Duration as D, given, *}, ExecutionContext.Implicits.*
 
   val f = Future(42)
   val r = Await.result(f, D.Inf)