Better indent in multi-line strings (KT-17849)

Inspired by MultilineStringEnterHandler from https://github.com/JetBrains/intellij-scala
https://github.com/JetBrains/intellij-scala/blob/edb741f344afff063197b406ca0c7c266a26c436/src/org/jetbrains/plugins/scala/editor/enterHandler/MultilineStringEnterHandler.scala

 #KT-17849 Fixed

idea/testData/editor/enterHandler/multilineString/spaces/dontInsertTrimMargin3.kt

@@ -0,0 +1,17 @@
+val a =
+  ("""
+      |  blah blah
+      |  blah blah
+   """ + """<caret>""").trimMargin()
+
+// TODO: Concatenation is not supported
+//-----
+val a =
+  ("""
+      |  blah blah
+      |  blah blah
+   """ + """
+       <caret>
+   """.trimIndent()).trimMargin()
+
+// TODO: Concatenation is not supported