Bring some utility functions to `fsspec.FUNC`

There are plenty of small utility functions that could be applied from a URI without having to instantiate a filespec object. It makes life much easier when you want to deal with different types of filesystem transparently.

Here are two examples:

def is_valid_file(path):
    """Check whether a file is a valid path"""
    mapper = fsspec.get_mapper(path)
    return mapper.fs.isfile(path)


def is_local_path(path):
    """Check whether a path point to a local filesystem."""
    mapper = fsspec.get_mapper(path)
    return mapper.fs.protocol == "file"

I guess the idea is similar to what fsspec.open already provides.

Start by reviewing the existing fsspec.open entry point and how the URI examples obtain filesystem objects through fsspec.get_mapper. Define which utility functions belong in fsspec.FUNC and how they should work across filesystem types; done means the requested URI-based utilities are available without instantiating a filesystem object.