refactor!: Introduce subpath exports and remove server-registry abstraction in loro-adaptors

[zxch3n]

This commit refactors the loro-adaptors package to improve modularity and
make peer dependencies truly optional.

Previously, importing from loro-adaptors would implicitly pull in all
peer dependencies, leading to "Could not resolve" errors if a user only
intended to use a subset of adaptors (e.g., only flock without loro-crdt).
Additionally, the server-registry abstraction was identified as unnecessary.

This change addresses these issues by:

• Introducing subpath exports for specific adaptor groups:
  • loro-adaptors/loro: Exports LoroAdaptor, LoroEphemeralAdaptor,
    LoroPersistentStoreAdaptor, EloAdaptor, and their server counterparts.
    This requires loro-crdt as a peer dependency.
  • loro-adaptors/flock: Exports FlockAdaptor and FlockServerAdaptor.
    This requires @loro-dev/flock as a peer dependency.
  • loro-adaptors/yjs: Exports YjsAwarenessServerAdaptor for Yjs awareness.
    This requires yjs as a peer dependency.
• Refactoring src/index.ts to only export general types and core utilities,
  effectively making it a lean entry point without direct adaptor implementations.
• Removing src/server/server-registry.ts and the AdaptorsForServer interface
  from src/types.ts, as this abstraction was deemed unnecessary.
• Updating package.json exports and tsdown.config.ts entry points to reflect
  the new modular structure.
• Updating README.md with revised installation instructions and usage examples
  for the new subpath imports.