builtin: add string.len_utf8() method (#14208)

vlib/builtin/string_test.v

@@ -24,6 +24,14 @@ fn test_add() {
 	assert a.ends_with('3')
 }
 
+fn test_len_utf8() {
+	assert 'Vlang'.len_utf8() == 5
+	assert 'María'.len_utf8() == 5
+	assert '姓名'.len_utf8() == 2
+	assert 'Слово'.len_utf8() == 5
+	assert 'Λέξη'.len_utf8() == 4
+}
+
 fn test_ends_with() {
 	a := 'browser.v'
 	assert a.ends_with('.v')