Partial style merging

[chbybnwr]

Describe the feature request

This can be resolved to the same style

const styles = stylex.create({
  foo: {
    color: {
      default: 'red',
      ':focus': 'blue'
    }
  },
  bar: {
    color: stylex.partial({
      ':focus': 'green'
    })
  }
})

stylex.props(styles.foo, styles.bar)

as this

const styles = stylex.create({
  baz: {
    color: {
      default: 'red',
      ':focus': 'green'
    }
  }
})

stylex.props(styles.baz)