Improve the flexibility of WorldProvider registration

WorldProviders now have the following requirements removed:
- __construct() is no longer required to have a specific signature
- static isValid() no longer needs to be implemented (you will still need it for registering, but it can be declared anywhere now)
- static generate() no longer needs to be implemented

This paves the way for more interesting types of world providers that use something other than local disk to store chunks (e.g. a mysql database).

WorldProviderManager no longer accepts class-string<WorldProvider>. Instead, WorldProviderManagerEntry is required, with 2 or 3 callbacks:
- ReadOnlyWorldProviderManager must provide a callback for isValid, and a callback for fromPath
- WritableWorldProviderManagerEntry must provide the same, and also a generate() callback

In practice, this requires zero changes to the WorldProviders themselves, since a WorldProviderManagerEntry can be created like this:
`new WritableWorldProviderManagerEntry(\Closure::fromCallable([LevelDB::class, 'isValid']), fn(string ) => new LevelDB(), \Closure::fromCallable([LevelDB::class, 'generate']))`

This provides identical functionality to before for the provider itself; only registration is changed.

src/world/format/io/WritableWorldProvider.php

@@ -24,14 +24,8 @@ declare(strict_types=1);
 namespace pocketmine\world\format\io;
 
 use pocketmine\world\format\Chunk;
-use pocketmine\world\WorldCreationOptions;
 
 interface WritableWorldProvider extends WorldProvider{
-	/**
-	 * Generate the needed files in the path given
-	 */
-	public static function generate(string $path, string $name, WorldCreationOptions $options) : void;
-
 	/**
 	 * Saves a chunk (usually to disk).
 	 */