Parse string unions as StringLiteralUnionTypeAnnotation (#46845)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46845

Previously, the parser was throwing away the values of a string union when storing it in the schema. It would only store the union as

```
{
  type: 'UnionTypeAnnotation',
  memberType: 'StringTypeAnnotation'
}
```

We now track the string literals through the parser and store them in the schema:

```
{
  type: 'StringLiteralUnionTypeAnnotation',
  types: [
    {
      type: 'StringLiteralTypeAnnotation'
      value: 'light'
    },
    {
      type: 'StringLiteralTypeAnnotation'
      value: 'dark'
    },
  ],
}
```

We aren't changing the generators, those still just output "string". They could eventually be made smarter.

The value of this is that the compat checker can now error when the union changes.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D63917685

fbshipit-source-id: 34a10e1f1910d2935837a3659f66049fd4473134