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

#KT-11704 Fixed

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralEnter-1.kt

@@ -0,0 +1,3 @@
+fun foo() {
+    val s = "fo<caret>something"
+}

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralEnter-2.kt

@@ -0,0 +1 @@
+// Empty file to satisfy test framework requirements

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralEnter.kt.after

@@ -0,0 +1,3 @@
+fun foo() {
+    val s = "foo.txt"
+}

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralTab-1.kt

@@ -0,0 +1,3 @@
+fun foo() {
+    val s = "fo<caret>something"
+}

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralTab-2.kt

@@ -0,0 +1 @@
+// Empty file to satisfy test framework requirements

idea/idea-completion/testData/handlers/multifile/FileRefInStringLiteralTab.kt.after

@@ -0,0 +1,3 @@
+fun foo() {
+    val s = "foo.txt"
+}