Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

idea/testData/refactoring/introduceVariable/stringTemplates/rawTemplateWithSubstring.kt.after

@@ -0,0 +1,9 @@
+fun foo(a: Int): String {
+    val s = """c$a
+    :${a + 1}d"""
+    val x = """_${s}_"""
+    val y = "_$a:${a + 1}d_"
+    val z = """_c$a:${a + 1}d_"""
+    val u = "_c$a\n:${a + 1}d_"
+    return """ab${s}ef"""
+}