feat(registry): make dependency installs deterministic (#318)

[maskedsyntax]

Added SHA256 hashing for dependencies to make sure installs are consistent.

• Now 'vix add' saves a content hash in vix.lock.
• 'vix deps' checks this hash to make sure nothing in the store was tampered with or corrupted.
• Updated 'vix store gc' to require a --project flag so it's harder to accidentally delete global cache entries.

Fixes #318.