KT-18040: Enable auto popup when typing after $e. in string template

#KT-18040 fixed

idea/idea-completion/testData/confidence/AutoPopupInStringTemplate.kt

@@ -0,0 +1,10 @@
+class Bar {
+    val aa = "Some"
+}
+
+fun foo() {
+    val bar = Bar()
+    val y = "$bar.<caret>"
+}
+
+// ELEMENT: aa