Reorganize codebase into multi-package mono-repo

[coderabbitai]

Context

As discussed in PR #272, the StaticHandler has a hard dependency on dart:io, which prevents Relic from running in WASM environments.

Proposal

Reorganize the codebase into a multi-package mono-repo structure. This would allow isolating platform-specific functionality:

• Create a relic_io package for functionality that depends on dart:io (e.g., StaticHandler)
• Keep the core Relic functionality platform-agnostic

Benefits

• Enable WASM compatibility for the core package
• Better separation of concerns
• Leverage Dart workspace support (SDK ^3.7.0)

References

• Original discussion: feat: Add IPAddress class (independent of dart:io) #272
• Requested by: @nielsenko