Make sure Scala 3 dialect is used when printing instrumented code

Previously, we would use the default dialect, which would print `import something._` instead of `import something.*` and this would break with the -source future flag that drops the former syntax.

Fixes #513

mdoc/src/main/scala-3/mdoc/internal/markdown/FailInstrumenter.scala

@@ -5,6 +5,7 @@ import java.io.PrintStream
 import scala.meta._
 
 final class FailInstrumenter(sections: List[SectionInput], i: Int) {
+  implicit val dialect: Dialect = dialects.Scala3
   private val out = new ByteArrayOutputStream()
   private val gensym = new Gensym()
   def instrument(): String = {

mdoc/src/main/scala-3/mdoc/internal/markdown/Instrumenter.scala

@@ -19,6 +19,7 @@ class Instrumenter(
     settings: Settings,
     reporter: Reporter
 ) {
+  implicit val dialect: Dialect = dialects.Scala3
   def instrument(): Instrumented = {
     printAsScript()
     Instrumented.fromSource(

tests/worksheets/src/test/scala/tests/worksheets/WorksheetSuite.scala

@@ -51,6 +51,19 @@ class WorksheetSuite extends BaseSuite {
        |""".stripMargin
   )
 
+  checkDecorations(
+    "import-future".tag(OnlyScala3),
+    """|import $scalac.`-source future`
+       |import scala.collection._
+       |val x = 1.to(4).toVector
+       |""".stripMargin,
+    """|import $scalac.`-source future`
+       |import scala.collection._
+       |<val x = 1.to(4).toVector> // : Vector[Int] = Vect...
+       |x: Vector[Int] = Vector(1, 2, 3, 4)
+       |""".stripMargin
+  )
+
   checkDiagnostics(
     "value-class",
     """|object Foo {