Code Insight: Support file path references inside of Kotlin string literals

#KT-11704 Fixed

idea/idea-completion/testData/basic/multifile/FileRefInStringLiteral/FileRefInStringLiteral.kt

@@ -0,0 +1,6 @@
+// NUMBER: 1
+// EXIST: foo.txt
+
+fun foo() {
+    val s = "fo<caret>"
+}

idea/idea-completion/testData/basic/multifile/FileRefInStringLiteralNoPrefix/FileRefInStringLiteralNoPrefix.kt

@@ -0,0 +1,8 @@
+// NUMBER: 3
+// EXIST: foo.txt
+// EXIST: bar.txt
+// EXIST: FileRefInStringLiteralNoPrefix.kt
+
+fun foo() {
+    val s = "<caret>"
+}