plots: extract rendering functionality to separate lib/submodule

[pared]

I think we need to discuss extracting dvc.plots.show to a separate lib.

Why?

1. dvc plots used to be only about Vega, and we made the user use a particular convention (saving plots as data files) in order to provide meaningful results in dvc plots show/diff commands.
2. turns out its hard to force user's to use only one convention - hence plots: introduce support for images #6431, and ideas like plots: Add support for plotly as backend for render plots dvc-render#7
3. loosening the plot's requirements made us focus the dvc commands on providing the data rather than a visualization as a whole, and allowed us to separate the HTML rendering from the Repo.plots methods
4. dvclive could use rendering too - but we don't want to add whole DVC for plotting purposes only
5. Seems like after plots: special chars handling for vega plots paths #6900 we will need to do something about generated HTML and js testing, which sounds flaky, so a separate testing suite might come in handy

Related: iterative/dvclive#61, #5980

cc @daavoo @dberenbaum