Initial plugin design

It's going to let you install and run models using https://github.com/mlc-ai/mlc-llm

e.g.

```pycon
from mlc_chat import ChatModule
from mlc_chat.callback import StreamToStdout
cm = ChatModule(model="Llama-2-7b-chat-hf-q4f16_1")
print(cm.generate(prompt="Three terrific names for a pet skunk"))
# Prints that out
print(cm.generate(prompt="Two more"))
# Prints out two more
```