Fix NormalizerRegistry.register()

[cpsievert]

Fixes (the currently internal) NormalizerRegistry.register() so that when a new strategy is registered, it is actually the first strategy to be considered. Discovered while investigating #1610. Here's a minimal example:

from shiny.express import ui
from shiny.ui._chat_normalize import BaseMessageNormalizer, message_normalizer_registry

class CustomResponseObject:
    def __init__(self):
        self.object = {"content": "response", "role": "assistant"}

class CustomResponseNormalizer(BaseMessageNormalizer):
    def normalize(self, message: CustomResponseObject):
        return message.object

    def can_normalize(self, message):
        return isinstance(message, CustomResponseObject)

    def normalize_chunk(self, chunk):
        pass

    def can_normalize_chunk(self, chunk):
        pass

message_normalizer_registry.register(
    "custom-response", CustomResponseNormalizer(), force=True
)

# Create and display empty chat
chat = ui.Chat(id="chat")
chat.ui()

@chat.on_user_submit
async def _():
    await chat.append_message(CustomResponseObject())