Preserve errors order when publishing diagnostics

The `sbt-server` was prepending a new probem and not appending.

The result was a `textDocument/publishDiagnostics` notification
containing a inverted list of problems compare to what was show in the
sbt console.

main/src/main/scala/sbt/internal/server/LanguageServerReporter.scala

@@ -51,7 +51,7 @@ class LanguageServerReporter(
     val pos = problem.position
     pos.sourceFile.toOption foreach { sourceFile: File =>
       problemsByFile.get(sourceFile) match {
-        case Some(xs: List[Problem]) => problemsByFile(sourceFile) = problem :: xs
+        case Some(xs: List[Problem]) => problemsByFile(sourceFile) = xs :+ problem
         case _                       => problemsByFile(sourceFile) = List(problem)
       }
     }