feat(typescript): Add support for explicit null in snippets

[amckinney]

This updates the dynamic snippet generators to support the new nullable container type introduced in #5601. With this, the dynamic snippets API will render null values when the property is nullable like so:

import { AcmeClient } from "acme";

async function main() {
    const client = new AcmeClient({
        environment: "https://api.example.com",
    });
    await client.users.create({
        username: "john.doe",
        tags: [
            "admin",
        ],
        metadata: {
            avatar: null,
            activated: null,
        },
        avatar: null,
    });
}
main();

[Swimburger]

TS part LGTM