Add Config File Support for Advanced Multiplexing

[ruffsl]

Summary:
Currently, CtrlAssist is configured via CLI arguments, which is simple but limits extensibility for more complex setups. Adding config file support would enable advanced scenarios, such as:

• Using more than two hardware controllers per virtual device
• Supporting multiple virtual devices, each with their own primary/assist pairings (e.g., for 2v2 player-vs-player with four controllers split into two pairs)
• Expressive, persistent, and shareable configuration for per game setups

Expected Behavior:
Users can specify controller mappings, mux modes, and other settings in a config file (e.g., TOML, JSON with comments, or YAML), allowing for more flexible and complex configurations than the CLI alone.

Current Behavior:
All configuration is done via CLI arguments, which is limited to simple use cases (e.g., only two controllers per virtual device, one virtual device at a time).

Format Considerations:

• TOML: Readable, widely used in Rust, but limited in expressiveness for advanced references.
• JSON (with comments): Familiar, but lacks native comments and references.
• YAML: Most expressive (anchors, references), but adds parsing complexity.
• Preference is for a format that balances readability, ease of parsing, and future extensibility.

Suggested Solution:

• Design a config schema for controller-to-virtual-device mapping, mux modes, and other options.
• Implement config file parsing (preferably TOML or JSON with comments; YAML only if necessary for advanced features).
• Allow CLI to override config file values for quick adjustments.

Notes:

• This would enable advanced multiplayer and accessibility scenarios.
• Consider documenting example configs for common setups.