Add xarray.show_backends alias

[VeckoTheGecko]

• Closes ENH: list_engines function #6577
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

Implemented according to #6577 (comment)

[headtr1ck]

Certainly would improve discoverability if top level API.

Not sure on the name, maybe backend is better than engine, but whether list or show....
Does anyone have a stronger preference on this?

[headtr1ck]

Suggested change

	def show_backends() -> dict[str, BackendEntrypoint]:
	show_backends = list_engines

Would be easier.

[VeckoTheGecko]

True, it would be easier, but the different name wouldn't be reflected in the docstrings or in the help information.

from xarray.backends import list_engines
show_backends = list_engines
print(help(show_backends))

gives

Help on _lru_cache_wrapper in module xarray.backends.plugins:

list_engines() -> 'dict[str, BackendEntrypoint]'
    Return a dictionary of available engines and their BackendEntrypoint objects.
    
    Returns
    -------
    dictionary
    
    Notes
    -----
    This function lives in the backends namespace (``engs=xr.backends.list_engines()``).
    If available, more information is available about each backend via ``engs["eng_name"]``.

Not sure if theres a standard approach to this sort of aliasing...