This pull request adds comprehensive support for Laravel-style validation rules for environment variables, with a focus on compatibility and extensibility. It introduces a modular structure for validation rule parsing and execution, implements new date validation rules, and provides macros for handling no-op and unsupported rules. The main highlights are:

Core features and structure:

• Introduced a new modular validation rule system, with themed modules for different rule types and a central dispatch function (parse_validation_rule) to handle rule parsing and instantiation.
• Added a crate-level documentation comment explaining the purpose, supported Laravel versions, and usage patterns for the validator library.

Date validation rules:

• Implemented support for Laravel's date validation rules, including date, date_format, after, after_or_equal, before, and before_or_equal, handling both literal dates and field references.

Laravel compatibility and unsupported rules:

• Added no-op implementations for Laravel rules that do not apply to scalar environment variables (e.g., array rules, input-shaping rules), ensuring they parse and are accepted for compatibility but have no effect.
• Introduced macros for defining both no-op rules and rules that are explicitly unsupported, providing clear error messages for rules that cannot be honored (e.g., file, image, exists).