json2: encode ascii chars < 0x20 in json (#12494)

vlib/x/json2/decoder_test.v

@@ -5,6 +5,13 @@ fn test_raw_decode_string() ? {
 	assert str.str() == 'Hello!'
 }
 
+fn test_raw_decode_string_escape() ? {
+	jstr := raw_decode('"\u001b"') ?
+	str := jstr.str()
+	assert str.len == 1
+	assert str[0] == 27
+}
+
 fn test_raw_decode_number() ? {
 	num := raw_decode('123') ?
 	assert num.int() == 123

vlib/x/json2/encoder.v

@@ -150,6 +150,9 @@ fn json_string(s string) string {
 				}
 			} else if chr == `"` || chr == `/` || chr == `\\` {
 				sb.write_string('\\' + chr.ascii_str())
+			} else if int(chr) < 0x20 {
+				hex_code := chr.hex()
+				sb.write_string('\\u00$hex_code')
 			} else {
 				sb.write_b(chr)
 			}

vlib/x/json2/encoder_test.v

@@ -5,6 +5,13 @@ fn test_json_string_characters() {
 	assert text.json_str() == '\\n\\r\\b\\f\\t\\\\\\"\\/'
 }
 
+fn test_json_escape_low_chars() {
+	esc := '\u001b'
+	assert esc.len == 1
+	text := json2.Any(esc)
+	assert text.json_str() == r'\u001b'
+}
+
 fn test_json_string() {
 	text := json2.Any('te✔st')
 	assert text.json_str() == r'te\u2714st'