The only reason the config() helper function existed was to access your design tokens in your CSS, like:
```css
.foo {
color: config('colors.blue')
}
```
Now that design tokens are nested in the new `theme` section, using the `config()` function is a bit more verbose:
```css
.foo {
color: config('theme.colors.blue')
}
```
This PR removes the `config()` function in favor of a new `theme()` function that is already scoped to the `theme` section of the config:
```css
.foo {
color: theme('colors.blue')
}
```
I can't think of any reason at all why you would need to access the non-theme values in your config from your CSS (like enabled variants, or your list of plugins), and the word `theme` is much more expressive than `config`, so I think this is a worthwhile change.
