feat: Enhanced Completion Providers with Values and Enum Support

[CodeWithKyrian]

This PR significantly enhances the completion provider system by adding built-in support for simple list-based and enum-based completions, making it much easier for developers to provide auto-completion without writing custom provider classes.

What's New

Enhanced CompletionProvider Attribute

The attribute now supports three mutually exclusive parameters:

• provider - Existing class-string or instance (backward compatible)
• values - Array of completion values for simple lists
• enum - Enum class-string for enum-based completions

// Simple list completion
#[CompletionProvider(values: ['draft', 'published', 'archived'])]
string $status

// Enum-based completion  
#[CompletionProvider(enum: StatusEnum::class)]
string $priority

// Custom provider (existing functionality)
#[CompletionProvider(provider: MyCustomProvider::class)]
string $category

Built-in Completion Providers

• ListCompletionProvider - Handles array-based completions with prefix filtering
• EnumCompletionProvider - Extracts values from string-backed, int-backed, and unit enums