`trim_messages` devx request

[sydney-runkle]

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From @baskaryan :)

would be nice if instead of

from langchain_core.messages import trim_messages
from langchain_core.messages.utils import count_tokens_approximately

trim_messages(..., token_counter=count_tokens_approximately)

we could do

from langchain_core.messages import trim_messages

trim_messages(..., token_counter="approx")