`frontend/svelte`: warn on ineffective style tag

[terrablue]

If the style tag in a Svelte component is empty or its rules do not style any of the tags in the same component, Svelte will remove it, resulting in an empty string style part. This previously led to an error, fixed in cfe7a92 (0.31) and cff72a2 (master).

In the future, we want to do even better and warn you when the style tag is effective (either empty, or none of its rules apply). This should be a buildtime warning.

cc @ralyodio