[Interactive] Include scope completions for synthic select tree

Previously, in case if query matches on existing class member
the returned completions were not returning symbols from scope.

Fixes the following case:
```scala
class Y {
  def bar: Unit =
   val argument: Int = 42
   arg@@   // should return both `local.argument` and `this.arg`
           // but tree looks like select -  `Select(This(Ident(Y)), Ident("arg"))`

  def arg: String = ???
}
```

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -112,6 +112,9 @@ object Completion {
     val completer = new Completer(mode, prefix, pos)
 
     val completions = path match {
+        case Select(qual @ This(_), _) :: _                    =>
+          val scope = if (qual.span.isSynthetic) completer.scopeCompletions else Map.empty
+          scope ++ completer.selectionCompletions(qual)
         case Select(qual, _) :: _                              => completer.selectionCompletions(qual)
         case Import(expr, _) :: _                              => completer.directMemberCompletions(expr)
         case (_: untpd.ImportSelector) :: Import(expr, _) :: _ => completer.directMemberCompletions(expr)

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -857,4 +857,17 @@ class CompletionTest {
         |}""".withSource
       .completion(m1, Set(("show",Method, "(using x$2: x$1.reflect.Printer[x$1.reflect.Tree]): String")))
   }
+
+  @Test def includeScopeForSyntheticThis: Unit = {
+    code"""|class Y() {
+           |  def bar: Unit = 
+           |    val argument: Int = ???
+           |    arg${m1}
+           |
+           |  def arg: String = ???
+           |}
+           |""".withSource
+      .completion(m1, Set(("arg", Method, "=> String"),
+                          ("argument", Field, "Int")))
+  }
 }