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/insertCustomMargin.kt

@@ -0,0 +1,9 @@
+class A {
+  val a = """blah blah<caret>""".trimMargin("#")
+}
+//-----
+class A {
+  val a = """blah blah
+      #<caret>
+  """.trimMargin("#")
+}