Combine LLM_*_PATH variables into one

[simonw]

These are getting messy:

llm/tests/conftest.py

Lines 29 to 31 in a597a9e

	monkeypatch.setenv("LLM_KEYS_PATH", str(keys_path))
	monkeypatch.setenv("LLM_LOG_PATH", str(log_path))
	monkeypatch.setenv("LLM_TEMPLATES_PATH", str(templates_path))

Especially since I now need to add yet another one for the path used for the file that stores the default model:

• Way to configure the default model #31

[simonw]

I'm going to call it LLM_USER_PATH and have it control the return value for this function:

llm/llm/cli.py

Lines 516 to 517 in a597a9e

	def user_dir():
	return pathlib.Path(click.get_app_dir("io.datasette.llm"))

[simonw]

Documentation: https://github.com/simonw/llm/blob/e1d2a71fd6692bf50831343b21a1b4587b8c59fb/docs/setup.md#custom-directory-location