Ensure --spacing-* variables take precedence over --container-* variables

[adamwathan]

Fixes #15146.

This PR updates the w-*, max-w-*, min-w-*, and basis-* utilities to make sure that --spacing-* values are preferred over --container-* values when there is a conflict.

Given this theme configuration:

@theme {
 --spacing-sm: 8px;
 --container-sm: 256px;
}

…utilities like max-w-sm will use 8px instead of 256px after this change.

Users can still be explicit about the value they want to use if they've introduced a naming collision like this by using our variable shorthand like max-w-(--container-sm).