Warn on automatic coercion to coordinate variables in Dataset constructor

[TomNicholas]

• Starts the deprecation cycle for Dataset constructor always coerces 1D data variables with same name as dim to coordinates #8959
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst
• Change existing code + examples so as not to emit this new warning everywhere.

[TomNicholas]

Tests will fail due to this new warning until I change all the constructors to use coords=...

[shoyer]

Hmm. I'm not sure how much adding vars as a new argument helps, given that we are issuing the exact deprecation warning in both cases.

Maybe we issue the warning only in the case where the user specifies data_vars, but not if they specify vars? Then the recommendation (if you see the warning) would either be to:

1. Move coordinate variables explicitly into coords, or
2. Use the vars keyword argument instead of data_vars

with both solutions silencing the warning.