Set target="_blank" for Source links in Scaladoc (#16282)

Context:
- erikerlandson/coulomb#368

This seems to be necessary so that the source links work with
javadoc.io. Note that Scaladoc 2 sets `target="_blank"`.

Author: armanbilge

scaladoc/src/dotty/tools/scaladoc/renderers/MemberRenderer.scala

@@ -72,7 +72,7 @@ class MemberRenderer(signatureRenderer: SignatureRenderer)(using DocContext) ext
 
   def source(m: Member): Seq[AppliedTag] =
     summon[DocContext].sourceLinks.pathTo(m).fold(Nil){ link =>
-      tableRow("Source", a(href := link)(m.sources.fold("(source)")(_.path.getFileName().toString())))
+      tableRow("Source", a(href := link, target := "_blank")(m.sources.fold("(source)")(_.path.getFileName().toString())))
     }
 
   def deprecation(m: Member): Seq[AppliedTag] = m.deprecated.fold(Nil){ a =>

scaladoc/src/dotty/tools/scaladoc/util/html.scala

@@ -108,6 +108,7 @@ object HTML:
   val onclick=Attr("onclick")
   val titleAttr =Attr("title")
   val onkeyup = Attr("onkeyup")
+  val target = Attr("target")
 
   def raw(content: String): AppliedTag = new AppliedTag(content)
   def raw(content: StringBuilder): AppliedTag = content