styleInject breaks tree-shaking

This function doesn't have a `/*#__PURE__*/` prefix, which breaks tree-shaking. Moreover, when I try to prepend it myself through `inject` option it gets stripped along with the entire string from the final bundle, which leaves you without CSS.

```js
function inject(cssVariableName) {
return (
'\n' +
`/*#__PURE__*/ (${styleInjectFork})(${cssVariableName}${
Object.keys(styleInjectOptions).length > 0
? `,${JSON.stringify(styleInjectOptions, function(_key, value) {
if (typeof value === 'function') {
return value.toString();
}
return value;
})}`
: ''
});`
);
}
```

In my case, I forked `styleInject`, because it's missing some features, so I had to compose it slightly different into the string (wrap it with brackets to turn it into expression, which is then invokable).