Add Dataset.subset() method for type-stable variable selection

[shoyer]

Addresses issue #3894 by providing a public API for selecting multiple variables that always returns a Dataset (unlike __getitem__) and accepts sequence types including tuples. This eliminates the need to convert tuples to lists when subsetting variables and provides better type stability for downstream code.

Unlike using __getitem__ with a list, an explicit method is more discoverable through IDE autocomplete and documentation.

• [x] Closes #3894
• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [x] New functions/methods are listed in api.rst

[keewis]

cc @eni-awowale