refactor validators to remove duplicate files

[gvwilson]

This PR replaces #5173 by rebasing @bmaranville's changes on top of main. Please see #5173 for detailed notes about the motivation and changes. The important changes are in:

• codegen/init.py

• codegen/datatypes.py

• codegen/validators.py

• doc/python/marker-style.md

• plotly/_subplots.py

• plotly/basedatatypes.py

• plotly/figure_factory/_annotated_heatmap.py

• plotly/io/_templates.py

• plotly/validator_cache.py

• 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).