chore(xplat): Add support for nested selectors to Griffel to StyleX converter

[behowell]

ℹ️ Note this is being merged into the xplat branch

Previous Behavior

The convertGriffelToStyleX platform adapter didn't support nested selectors.

New Behavior

The converter now adapts the griffel format of nested selectors:

{
  color: 'red',
  ':hover': {
    color: 'green',
    '@media (forced-colors: active)': {
       color: 'highlighttext',
     },
  },
}

into the stylex format of nested selectors:

{
  color: {
    default: 'red',
    ':hover': {
      default: 'green',
      '@media (forced-colors: active)': 'highlighttext',
    },
  },
}

[JasonVMo]

Good catch on this one, I hadn't noticed the signature differences between stylex and griffel for these.

[JasonVMo]

Interesting, didn't realize index signatures could be added this way.

[JasonVMo]

If I follow the logic now, we will emit warnings with unsupported selectors but not end up passing them to stylex. I think that is better than having the hard errors.

[behowell]

Yeah, we'll need a longer-term solution to catch and fix these cases at development time, like a lint rule or something. But for now, this at least makes it so it doesn't crash with an invalid selector.