Add llm plugins --all option to view default plugins, as seen in Datasette

[simonw]

Following:

• Environment variable to disable plugins #256

I disabled the display of default plugins there. I should re-enable it if you pass --all

Here's the relevant Datasette help output:

% datasette plugins --help
Usage: datasette plugins [OPTIONS]

  List currently installed plugins

Options:
  --all                    Include built-in default plugins
  --requirements           Output requirements.txt of installed plugins
  --plugins-dir DIRECTORY  Path to directory containing custom plugins
  --help                   Show this message and exit.