Added socket listening

[shitohana]

Description

This PR implements Unix socket support for the Helix editor, addressing #387. It enables external processes to send commands to a running Helix instance via a Unix socket, improving automation and integration capabilities. Since a plugin system is not planned for near-term addition, this feature provides a practical way to communicate with Helix from other processes (e.g., opening files from tools like Yazi or Broot).

Changes

1. Added Unix socket listener at /tmp/helix.sock to receive commands.
2. Integrated socket handling into the event loop to process incoming messages as MappableCommands.
3. Included error handling for socket binding, reading, and command parsing, with user feedback via editor status messages.
4. Updated Application struct to include a socket_rx channel for receiving socket messages.
5. Used tokio::net::UnixListener and tokio::sync::mpsc for asynchronous socket communication.

Why Merge

• Resolves Support RPC through a socket. #387, enabling external command integration.
• Enhances editor extensibility for scripting and automation.
• Maintains stability with robust error handling and minimal changes to existing code.
• Unix-only implementation ensures compatibility without affecting other platforms.

Potential Improvements

• The socket path (/tmp/helix.sock) is currently hardcoded. Making it configurable via the Helix config file would improve flexibility and avoid potential conflicts.
• Currently this works only on unix systems.
• Add tests

Diff Summary

• Modified helix-term/src/application.rs to include Unix socket setup, listener, and command handling.
• Added socket_rx to Application struct and integrated it into the event loop.
• Implemented handle_socket_command to parse and execute commands with error reporting.

[shitohana]

Demo:

[idealseal]

I think this should use XDG_RUNTIME_DIR as the socket path by default, see https://man.archlinux.org/man/file-hierarchy.7.en#RUNTIME_DATA and https://specifications.freedesktop.org/basedir-spec/latest/#variables.