builtin: add a contains_only method on string (#14830)

vlib/builtin/string_test.v

@@ -447,6 +447,13 @@ fn test_contains_any() {
 	assert !''.contains_any('')
 }
 
+fn test_contains_only() {
+	assert '23885'.contains_only('0123456789')
+	assert '23gg885'.contains_only('01g23456789')
+	assert !'hello;'.contains_only('hello')
+	assert !''.contains_only('')
+}
+
 fn test_contains_any_substr() {
 	s := 'Some random text'
 	assert s.contains_any_substr(['false', 'not', 'rand'])