Umbrella: Deprecate legacy library code

[EthanThatOneKid]

What's your idea?

Background

Mega-PR #577 left behind some libraries containing legacy code (/src/lib/public/legacy).

Plan

• [x] #582
• [ ] Find a long-term location for /src/lib/public/legacy/theme.ts
• [ ] Replace Spacing.svelte with Block.svelte