feat: add Identifier type for Python bindings

[wu-vincent]

Motivation

Identifier<T> is used throughout the API (dimensions, block types, actor types, enchantments, etc.) but was exposed to Python as a plain str. This had two problems:

1. No structured access — plugin developers had to manually parse "namespace:key" strings to extract components.
2. Weak type safety — type checkers couldn't distinguish between a DimensionId and an ActorTypeId since both were just str. You could pass Dimension.OVERWORLD to spawn_actor(type=...) without any type error.

With Identifier[T], type checkers can now catch misuse:

# Type checker ERROR: Expected Identifier[ActorType], got Identifier[Dimension]
dimension.spawn_actor(location, Dimension.OVERWORLD)

# Type checker OK:
dimension.spawn_actor(location, ActorType.ZOMBIE)

Before

dim_id = dimension.id          # "minecraft:overworld" (str)
type(dim_id)                   # <class 'str'>
# To get namespace/key, you had to parse manually:
ns, key = dim_id.split(":")    # "minecraft", "overworld"

After

dim_id = dimension.id          # Identifier(minecraft:overworld)
type(dim_id)                   # <class 'Identifier'>
dim_id.namespace               # "minecraft"
dim_id.key                     # "overworld"

# Backward compatible — still compares with str:
dim_id == "minecraft:overworld"  # True
str(dim_id)                      # "minecraft:overworld"

# Accepts str input everywhere:
level.get_dimension("overworld")              # still works
level.get_dimension(Dimension.OVERWORLD)      # also works

Summary

• Add PyIdentifier struct to expose Identifier<T> as a proper Python object with namespace and key properties
• Update Identifier<T> type caster to return Identifier objects instead of str, while still accepting both str and Identifier as input
• Fix __str__ methods on BlockType, Enchantment, and ItemType to return str (not Identifier)
• Add Identifier generic facade in stubs and stubgen for Identifier[T] type hints
• Validate that namespace and key are non-empty in PyIdentifier constructors

Test plan

• Build and verify compilation
• Run Python tests (pytest tests/endstone_test)
• Verify Dimension.OVERWORLD.namespace == "minecraft" and .key == "overworld"
• Verify Dimension.OVERWORLD == "minecraft:overworld" backward compat
• Run stubgen and verify generated stubs