Environment variable to disable plugins

[simonw]

Would help for when a plugin in development breaks and prevents llm uninstall from working.

Imitate this:

• Ability to only load a specific list of plugins datasette#2164

[simonw]

Here's how the test for that works: simonw/datasette@6bfe104#diff-4c2f3d8926fa53ec1181bfea8f3416584b34eade24fb0386581cb1c6acae1d76

[simonw]

Documentation for plugin authors can mention this too.

[simonw]

Manual testing:

$ llm plugins                               
[
  {
    "name": "llm-gpt4all",
    "hooks": [
      "register_models"
    ],
    "version": "0.1.1"
  },
  {
    "name": "llm-claude",
    "hooks": [
      "register_models"
    ],
    "version": "0.4.0"
  },
  {
    "name": "llm-mlc",
    "hooks": [
      "register_commands",
      "register_models"
    ],
    "version": "0.5"
  },
  {
    "name": "llm-sentence-transformers",
    "hooks": [
      "register_commands",
      "register_embedding_models"
    ],
    "version": "0.1"
  },
  {
    "name": "llm-clip",
    "hooks": [
      "register_embedding_models"
    ],
    "version": "0.1"
  },
  {
    "name": "llm-cluster",
    "hooks": [
      "register_commands"
    ],
    "version": "0.2"
  },
  {
    "name": "llm.default_plugins.openai_models",
    "hooks": [
      "register_commands",
      "register_embedding_models",
      "register_models"
    ]
  }
]
$ LLM_LOAD_PLUGINS='' llm plugins           
[
  {
    "name": "llm.default_plugins.openai_models",
    "hooks": [
      "register_commands",
      "register_embedding_models",
      "register_models"
    ]
  }
]
$ LLM_LOAD_PLUGINS='llm-cluster' llm plugins
[
  {
    "name": "llm-cluster",
    "hooks": [
      "register_commands"
    ],
    "version": "0.2"
  },
  {
    "name": "llm.default_plugins.openai_models",
    "hooks": [
      "register_commands",
      "register_embedding_models",
      "register_models"
    ]
  }
]
$ LLM_LOAD_PLUGINS='llm-cluster,llm-mlc' llm plugins
[
  {
    "name": "llm-cluster",
    "hooks": [
      "register_commands"
    ],
    "version": "0.2"
  },
  {
    "name": "llm-mlc",
    "hooks": [
      "register_commands",
      "register_models"
    ],
    "version": "0.5"
  },
  {
    "name": "llm.default_plugins.openai_models",
    "hooks": [
      "register_commands",
      "register_embedding_models",
      "register_models"
    ]
  }
]