[feature] `stylex.env` to provide constants and utility functions to StyleX

## Inspiration

`process.env` is used for environment variables which are essentially constants.

`stylex.env` should work similarly. It should be possible to provide constants to the babel as configuration which will be inlined during compilation, before other stylex transforms.

This would lead to a zero cost way to have shareable constants and even utility functions.

---

We are also considering another API to allow configuring entire packages as "constant providers" in #1355. We could start off by offering our own package of utilities:

```tsx
import * as stylex from '@stylexjs/stylex';
import {opacity, colorMix} from '@stylex/utils';
import {vars} from './myTheme.stylex';

const styles = stylex.create({
button: {
backgroundColor: colorMix(vars.accent, 'black', 0.2), // === `color-mix(${vars.accent} 20%, black)`
boxShadow: `0 4px 4px 2px ${opacity(vars.accent, 0.35)}` // === `0 4px 4px 2px color-mix(${vars.accent} 35%, transparent)`,

}
});
```