Move IFileSystem out of boards.py

[nathanjmcdougall]

As per

pins-python/pins/drivers.py

Lines 9 to 10 in d215a8c

	# TODO: move IFileSystem out of boards, to fix circular import
	# from .boards import IFileSystem

I propose moving IFileSystem into its own pins/fs.py file. It would still imported in boards.py for type hinting, so this wouldn't break backward compatibility.

It could then be used for type annotations in drivers.py without a circular import. This is currently the only other place where it is referenced.