Move imports from global into package-specific deno.json files

• an attempt to make @nostrify/* packages dependencies be installed automatically (and fix #28 (closed))
• I am new to jsr and deno, not sure if the import section in the global deno.json should be kept or it's ok to delete. Probably it's better to delete to avoid redundancy.
• Could not test end to end as I have no jsr repo and did not succeed in local linking and --import-map tweaking.

Summary by CodeRabbit

• Chores
  • Standardized Deno import mappings across packages to improve dependency resolution and build consistency.
  • Added mappings for common utilities and libraries (e.g., standard assert/encoding/json/streams, caching, websockets, schema validation, cryptography, and Nostr tools).
  • Introduced package-level import maps and removed a central mapping to reduce coupling and clarify dependencies.
  • No changes to public APIs or runtime behavior; these updates are configuration-only and aim to enhance reliability for Deno-based workflows.