plots: templates option is clunky

yes, it’s more about this command’s interface in this case. Feels like this list {simple,linear,confusion,confusion_normalized,scatter,smooth} will be growing / changing. Why do we have to specify it on the parser level, it’s not very common to limit that large number of targets there.

_Originally posted by @shcheklein in https://github.com/iterative/dvc.org/pull/3691#discussion_r907907003_

[daavoo]

iterative/dvc-render#8 is an example where we would have to update the list of templates in a few places?

I think we should just replace the hardcoded names with something like :

from dvc_render.vega_templates import TEMPLATES
TEMPLATE_CHOICES = [x.default_name for x in TEMPLATES] 

[pared]

I think that the simplest seems to be just list available templates on dvc plots templates and dvc plots templates {name} would print to stdout. No dumping all templates, no .dvc/plots behavior - just let user do whatever they want. That is simple and easy to understand.