Implement a validator registry

[sortedcord]

As it stands, the VALIDATORS dict is hardcoded, which limits modularity. If another developer wants to add custom settings and validators (e.g., from a plugin or extension), they'd need to modify the core code, which defeats modular design.

• Create a global validator registry with a simple API
• Make it so that extensions can use the register_validator decorator for their setting validation
• Make changes to the core validators to make use of the decorator as well.