Upgrade to nikic/fast-route v2

[philipobenito]

Upgrade the FastRoute dependency from ^1.3 to v2 when it reaches stable.

Current state:

• league/route depends on nikic/fast-route ^1.3 (released 2018)
• FastRoute v2.0.0-beta1 was released March 2024 but has not progressed to stable

What v2 changes:

• dispatch() returns Matched|NotMatched|MethodNotAllowed result objects instead of arrays
• RouteCollector replaced by ConfigureRoutes interface
• GroupCountBased dispatcher marked as @final (breaks our current inheritance)
• Built-in GenerateUri interface for reverse routing
• Built-in Cache interface
• PHP 8.1+ minimum with strict types throughout

Required work:

• Replace Dispatcher inheritance with composition (the Dispatcher currently extends GroupCountBased)
• Adapt to new result object types in matchRequest() and dispatchRequest()
• Update Router::prepareRoutes() for the new ConfigureRoutes API
• Potentially leverage v2's built-in URI generation for reverse routing (Reverse routing / URL generation from named routes #355)
• Potentially leverage v2's caching alongside our index-based approach

Prerequisite: FastRoute v2 reaching a stable release.

Target: 8.0