Add "Too long character literal -> string" fix to ILLEGAL_ESCAPE error

So #KT-23788 Fixed

idea/testData/quickfix/tooLongCharLiteralToString/backslash.kt

@@ -0,0 +1,7 @@
+// "Convert too long character literal to string" "false"
+// ACTION: Introduce local variable
+// ERROR: Illegal escape: ''\''
+
+fun foo() {
+    '\'<caret>
+}

idea/testData/quickfix/tooLongCharLiteralToString/startWithBackslash.kt

@@ -0,0 +1,6 @@
+// "Convert too long character literal to string" "true"
+// ERROR: Illegal escape: '\ '
+
+fun foo() {
+    '\ bar<caret>'
+}

idea/testData/quickfix/tooLongCharLiteralToString/startWithBackslash.kt.after

@@ -0,0 +1,6 @@
+// "Convert too long character literal to string" "true"
+// ERROR: Illegal escape: '\ '
+
+fun foo() {
+    "\ bar"
+}