x.json2: create custom scanner for scanning JSON (#8716)

vlib/x/json2/decoder_test.v

@@ -54,8 +54,16 @@ fn test_raw_decode_null() {
 
 fn test_raw_decode_invalid() {
 	json2.raw_decode('1z') or {
-		assert err == '[json] invalid JSON. (0:0)'
+		assert err == '[x.json2] invalid token `z` (0:17)'
 		return
 	}
 	assert false
 }
+
+fn test_raw_decode_string_with_dollarsign() {
+	str := json2.raw_decode(r'"Hello $world"') or {
+		assert false
+		json2.Any{}
+	}
+	assert str.str() == r'Hello $world'
+}