Improve and expand extra_model_paths.yaml

[vvuk]

This PR expands out what can be done in extra_models_paths.yaml:

• Adds base_paths in addition to base_path, to allow specifying multiple paths to prepend
• Allows overriding the models_dir, output_directory, etc.
• Allows explicitly specifying paths without base_path handling
• Fixes/changes order of expandvars/expanduser so that ~ can be used in environment variables

I've written docs directly into extra_model_paths.yaml.example, but reproducing here:

# Config for ComfyUI
#
# In all paths below:
#    - ~ will expand out to the current user's home directory (typically C:\Users\username on Windows,
#      /home/username on Linux, /Users/username on MacOS).
#    - $ENV_VAR will expand out to the value of the environment variable ENV_VAR. However, if
#      ENV_VAR is not set, the path will be ignored ($ENV_VAR will not expand out to an empty string).
#    - environment variables are expanded before ~, so ENV_VAR=~/foo will work properly
#    - if a path beings with @ it is treated as an explicit path, and no base_path will be
#      prepended to it (but environment variable and ~ expansion still apply)
#
comfyui:
    # If is_default is set to true, all the directories below will be prepended
    # instead of appended to the default ComfyUI lists.
    # instead of appending
    is_default: true

    # If base_path is specified, all the directories referenced below will be
    # relative to this path. For example, if you want to move comfy's models etc
    # to ~/comfy_data, set base_path to ~/comfy_data
    #base_path: ~/comfy_data

    # If base_paths is specified, it's treated as a multiple set of base_paths. Each
    # path in following entries is appended to each element of base_paths. If base_path is
    # also present, it will be included as the first element.
    #
    # This is useful to be able to easily specify multiple roots -- for example,
    # you could have one /shared_comfy_data that is a large network mount with
    # models shared amongst many users/machines, and then ~/local_comfy_data
    # where you can place models you're locally experimenting with. Specifying
    #   base_paths: ~/local_comfy_data /shared_comfy_data
    # will let Comfy search both paths.
    #
    #base_paths: |
    #    $LOCAL_COMFY_ROOT
    #    $COMFY_ROOT

    # The singular models/output/temp/input/user directories can be overriden as well.
    # These are single paths; they can't be a list. Base path handling still applies, but the
    # first directory that exists will be used. If none exist, the value isn't touched.
    #
    # For example, if base_paths is set to "~/local_comfy_data /shared_comfy_data",
    # and output_directory is set to "output", Comfy will look for for ~/local_comfy_data/output
    # first. If it exists, it will be used as the output directory. If models_dir is set to "models",
    # Comfy will look for ~/local_comfy_data/models -- if that doesn't exist, it will look for
    # /shared_comfy_data/models next.

    # The `models_dir`. This is used by some custom nodes to find the "models" directory
    # into which to place downloads (e.g. CogVideoX does this).
    #models_dir: models
    #output_directory: output
    #temp_directory: temp
    #input_directory: input
    #user_directory: user

    # Where to look for custom nodes. The @custom_nodes ensures that the custom_nodes
    # directory relative to the comfy install will be searched. The custom_nodes without
    # the @ means a directory called custom_nodes will also be searched in each of the base
    # paths.
    custom_nodes: |
        @custom_nodes
        custom_nodes

    # Different model types in use by Comfy. These are the defaults.
    # If you want the "base path" handling you _must_ respecify these here,
    # even if you want to keep the default suffix values. Specifying
    # just base_path (or base_paths) is not enough.
    #
    # Other model types can be added to this list, and some Comfy custom
    # nodes may add their own model types.
    checkpoints: models/checkpoints
    clip: models/clip
    text_encodings: models/text_encodings
    clip_vision: models/clip_vision
    configs: models/configs
    controlnet: models/controlnet
    diffusion_models: |
                  models/diffusion_models
                  models/unet
    embeddings: models/embeddings
    loras: models/loras
    upscale_models: models/upscale_models
    vae: models/vae