YAML stringify not support emoji flags.

**Describe the bug**

YAML parse not support emoji flags.

**Steps to Reproduce**

```ts
import * as YAML from "https://deno.land/std@0.203.0/yaml/mod.ts";
const yamlStr = `
country: 🇯🇵
`;

const data = YAML.parse(yamlStr) as any;

console.log(data); // { country: "🇯🇵" }

console.log(YAML.stringify(data)); // country: "\U0001F1EF\U0001F1F5"
```


**Expected behavior**

support emoji flags.

**Environment**

deno 1.37.0 (release, x86_64-unknown-linux-gnu)
v8 11.8.172.3
typescript 5.2.2