Not tree shaking styles during dev mode

### Describe the issue

when there are multiple stylex.defineVars in a file all of them are added to the CSS in dev mode even if not all are imported

### Expected behavior

tree shaking correctly removes unused `stylex.defineVars` calls

### Steps to reproduce

index.tsx
```tsx
import * as stylex from "@stylexjs/stylex";
import { Pink } from "./variable.stylex";

const styles = stylex.create({
color: {
color: Pink.pink,
},
});

export function Playground() {
return (

);
}
```

variable.stylex.ts
```ts
import * as stylex from "@stylexjs/stylex";

export const Pink = /*@__PURE__*/ stylex.defineVars({
pink: "pink",
});

export const Green = /*@__PURE__*/ stylex.defineVars({
green: "green",
});
```

`Green` will end up being included in the css in dev mode, and thus the extrenous `--green-` variable will end up being included.

### Test case

run `bun install`
run `bun run dev`

(though any package manager should work)

https://github.com/Mkassabov/stylex-vite-tree-shaking

observe that the green variable has been added

### Additional comments

adding `/*@__PURE__*/` to signify these shouldn't be included in dev mode also doesn't work

The main reason for wanting this is we add our entire color system (which is some ~7500 colors) as stylex styles. the vast majority of which aren't used; this causes the chrome dev tools to become extremely slow