Fix facet_col and animation_frame in px.imshow for xarrays.

[k-rl]

Pull request addressing #4329 and #4330.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[alexcjohnson]

Would you create a new ## UNRELEASED section above ## [5.16.1] and move these there? They can also be combined and reference the PR, something like "Fixed two issues with px.imshow: [#4330], facet_col is an earlier dimension than animation_frame for xarrays, and [#4329], facet_col has string coordinates in xarrays [#4331]"

[alexcjohnson]

💃 excellent fix @Karl-Krauth - and beautiful tests! Just a tweak to the changelog and we'll be good to merge 🥇

[k-rl]

Done :)