Rewrite typed_ops

[headtr1ck]

• Related to mypy does not understand output of binary operations #7780
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

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(Looks great so far!)

[headtr1ck]

Basically all type: ignores in the new tests are currently broken.
Some numpy ones are expected because e.g. np.ndarray.__add__ is typed wrong and assumes that you get a np.ndarray if you do np.array() + xr.DataArray(), which is wrong. (See #7780)

Why some of the python type ops are failing I have to investigate.

[headtr1ck]

Wow, this turns out to be more tricky than I thought.
E.g. Variable(...) + DataArray(...) returns a Variable type. This is because np.typing.ArrayLike allows _SupportsArray, which both, variables and DataArrays are. Therefore in the typing of Variable.__add__, mypy thinks that DataArrays are allowed.

I really need a way to specify that we support ArrayLike but not DataArrays.....
Maybe we could hack it back like before and add an overload for DataArrays to return DataArray, which is not really correct but effectively resolves this problem.

[headtr1ck]

Now this PR brings the static typing of the ops methods to the best we can currently achieve.
Two mayor open points remain (including the one of the original issue) but they are upstream problems and we cannot fix those.

1. Numpy: they type basically all their ops wrong. Whenever you do np.array() + xr.DataArray() you will get a np.ndarray according to mypy. The only workaround is to reverse the order, e.g. do xr.DataArray() + np.array(). Numpy needs to change their typing in a way that classes with __array_function__ take precedence (e.g. return NotImplemented/NoReturn) and classes with __array__ return np.ndarray.
2. Typeshed: basic types like int or list pretend to accept anything, which is defacto not true. For non-compatible types they should return NotImplemented as well. See https://discuss.python.org/t/make-type-hints-for-eq-of-primitives-less-strict/34240

The added "unit" tests (basically only checks for mypy) have type ignores for these cases, so we will realize when something upstream has changed.

[headtr1ck]

This is the issue that is also causing this problem here: #4601

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Awesome job @headtr1ck ! Very impressive, thank you.