feat: better support for reasoning/thinking

[cpsievert]

Pairs with tidyverse/ellmer#815

Partially addresses #59. To fully close there we'll want to:

1. Allow .stream() to stream thinking deltas (so you can differentiate between thinking/non-thinking text)
2. Come up with a better display of thinking text for .chat()

import chatlas as ctl

chat = ctl.ChatAnthropic(model="claude-haiku-4-5-20251001", reasoning=1200)
chat = ctl.ChatGoogle(reasoning=1200)
chat = ctl.ChatOpenAI(model="gpt-5", reasoning="low")

chat.chat("Tell me a joke. Think hard.", echo="none")
chat.chat("Why is that funny?", echo="none")

print(repr(chat.get_turns()))