Create experimental APIs for nested `defineVars`, `defineConsts`, and `createTheme` APIs

It's common for engineers to want to "nest" design tokens under one object. We want to create experimental APIs that will allow for such patterns.

```
stylex.unstable_defineVarsNested({foo: {...}, bar: {...}})
stylex.unstable_defineConstsNested
stylex.unstable_createThemeNested
```

We would need a separate API to handle conditional styles in such cases