Switch from appdirs to click.get_app_dir()

[simonw]

When building https://githb.com/simonw/llm I realized I don't need appdirs because Click has this already: https://click.palletsprojects.com/en/8.1.x/api/#click.get_app_dir

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

https://github.com/simonw/llm/blob/cb41409e2b499b76ec37db178c3b0fc042d59129/llm/__init__.py#L104

[simonw]

I'm torn between io.datasette.dclient and io.datasette - I'm going to use the former, just because it's most consistent with io.datasette.llm.