Extract Function/Introduce Property: Support extraction of string template fragments

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

@@ -0,0 +1,13 @@
+// SUGGESTED_NAMES: s, getX
+// PARAM_DESCRIPTOR: value-parameter val a: kotlin.Int defined in foo
+// PARAM_TYPES: kotlin.Int
+fun foo(a: Int): String {
+    val x = """_${s(a)}_"""
+    val y = "_$a:${a + 1}d_"
+    val z = """_c$a:${a + 1}d_"""
+    val u = "_c$a\n:${a + 1}d_"
+    return """ab${s(a)}ef"""
+}
+
+private fun s(a: Int) = """c$a
+    :${a + 1}d"""