Publish JSON/YAML configuraion schemas

It would be very helpful to provide an official JSON Schema for lsd's configuration file, as described in the configuration documentation.

The schema could either be:

• Maintained within this repository, or
• Published via SchemaStore (see the schemastore repository)

Why does this matter?

Providing a JSON Schema would significantly improve the configuration experience by:

• Enabling real-time validation in editors
• Reducing configuration errors
• Improving discoverability of available options
• Minimizing debugging time caused by invalid settings

Possible approach

One potential implementation could leverage existing libraries such as https://docs.rs/jsonschema/latest/jsonschema/