Specifies the possible value types for form fields.

Defines the structure for a validation rule in the form system. A ValidationRule is a two-part array containing:

• A validation condition (as boolean, RegExp, number, function, etc.)
• An optional custom error message to show when validation fails

The first element's type depends on the specific rule:

• For boolean rules (like 'required'): true/false
• For pattern rules (like 'email', 'regex'): RegExp object
• For range rules: number or number[] (min/max values)
• For equality checks: string (field name to compare with)
• For custom validation: () => boolean function

const requiredRule: ValidationRule = [true, 'This field is required'];