[FEAT] Support for Custom Godot Build Installation via SymLink

[imLymei]

Description

Currently, GodotEnv supports installing and managing official Godot versions, but doesn't have a way to register and use custom-built Godot executables (like GodotSteam) as the global Godot executable via the symlink mechanism.

This would extend GodotEnv's existing Godot version management capabilities to include custom builds, which would be particularly helpful for developers working with Steam integrations or other specialized Godot builds.

Use Case

I want to install my custom GodotSteam build as the "global" Godot executable through GodotEnv's symlink system, allowing me to use it as the default Godot environment while still being able to easily update or switch between it and official builds.

Proposed Solution

Add functionality to:

1. Register a custom Godot build from a local path
2. Allow the custom build to be "used" via the existing symlink mechanism (godotenv godot use)
3. Update the $GODOT environment variable to point to the custom build when it's active
4. (Optional enhancement) Support remote download and installation of common custom builds like GodotSteam

Example Command

# Register a custom build
godotenv godot register --name "godot-steam-4.2" --path="/path/to/my/GodotSteam.exe"

# Use the custom build
godotenv godot use godot-steam-4.2

Benefits

• Seamless integration of custom Godot builds into the existing version management workflow
• Ability to switch between official and custom builds using the same commands
• Maintains the convenience of the $GODOT environment variable pointing to the active build