Rename `_implementation`, `_backend_version` -> `implementation`, `backend_version`

[dangotbanned]

I'd already started doing this for namespaces and was reminded of the inconsistency in (#3127 (comment))

import narwhals as nw

series = nw.Series.from_iterable("", [1, 2, 3], backend="pandas")
>>> series.implementation is series._compliant._implementation is series._compliant.str.implementation
True

I'm mainly focused on implementation, but backend_version is relevant to (#3127).

version might be worth considering as well, I'd already made that change in (#2572):

narwhals/narwhals/_plan/protocols.py

Lines 91 to 98 in 53d314d

	# NOTE: Unlike the version in `nw._utils`, here `.version` it is public
	class StoresVersion(Protocol):
	_version: Version
	@property
	def version(self) -> Version:
	"""Narwhals API version (V1 or MAIN)."""
	return self._version