Allow loading js extensions without copying to /web folder

[pythongosssss]

Allows serving web content from the custom_nodes folder, meaning that custom nodes dont have to symlink/copy web files.
Also helps when people remove custom nodes, they often leave web files behind and have weird issues.

The supports = ["custom_nodes_from_web"] allows custom node authors to check if the current version of ComfyUI the user is running supports this feature or not, e.g. they couyld implement it something like this:

def should_install_js():
    return not hasattr(PromptServer.instance, "supports") or "custom_nodes_from_web" not in PromptServer.instance.supports

If there is a better mechanism for this, open to suggestions.

A simple test for this:

NODE_CLASS_MAPPINGS = {
}

WEB_DIRECTORY = "./somejs"

__all__ = ['WEB_DIRECTORY']

Then create somejs/hello.js in the custom_nodes folder with alert("hello")