FoldExpressionIntoStream: disable for non-string separator and automatically convert char

CharToStringIntention#stringForCharLiteral moved to PsiLiteralUtil
Fixes IDEA-195621 "Expression can be folded into Stream chain" produces red code when delimiter is char type

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/afterConcatCharDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "true"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = String.join("_", a, b, c);
+    return s;
+  }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/beforeConcatCharDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "true"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = a + <caret>'_' + b + '_' + c ;
+    return s;
+  }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/beforeConcatIntDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "false"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = a + <caret>5 + b + 5 + c ;
+    return s;
+  }
+}