TypeScript should allow undefined in args.default

[jcalfee]

Environment

citty ^0.1.6

Reproduction

const c = defineCommand({
  args: {
    accountId: {
      type: 'string',
      description: '[env: ACCOUNT_ID]',
      default: process.env.ACCOUNT_ID,
      required: true
    },
// ...

Describe the bug

I have environment variables and I would like to pass them though the args.default so the CLI has a chance to change the values. When I do that, process.env.ANY_VAR could be string | undefined. This causes a typescript error when I try to use that in citty.

Additional context

No response

Logs

[jcalfee]

Possibly related #132