Partial style merging

### Describe the feature request

This can be resolved to the same style
```js
const styles = stylex.create({
foo: {
color: {
default: 'red',
':focus': 'blue'
}
},
bar: {
color: stylex.partial({
':focus': 'green'
})
}
})

stylex.props(styles.foo, styles.bar)
```
as this
```js
const styles = stylex.create({
baz: {
color: {
default: 'red',
':focus': 'green'
}
}
})

stylex.props(styles.baz)
```