Return error for string literal beginning with single quote (#2964)

In order for a string literal to be valid in JSON, the value must be
surrounded by double quotes.

src/jv_parse.c

@@ -512,6 +512,8 @@ static pfunc check_literal(struct jv_parser* p) {
   switch (p->tokenbuf[0]) {
   case 't': pattern = "true"; plen = 4; v = jv_true(); break;
   case 'f': pattern = "false"; plen = 5; v = jv_false(); break;
+  case '\'':
+    return "Invalid string literal; expected \", but got '";
   case 'n':
     // if it starts with 'n', it could be a literal "nan"
     if (p->tokenbuf[1] == 'u') {

tests/jq.test

@@ -2187,6 +2187,9 @@ try ["ok", setpath([1]; 1)] catch ["ko", .]
 {"hi":"hello"}
 ["ko","Cannot index object with number"]
 
+try fromjson catch .
+"{'a': 123}"
+"Invalid string literal; expected \", but got ' at line 1, column 5 (while parsing '{'a': 123}')"
 
 # ltrimstr/1 rtrimstr/1 don't leak on invalid input #2977