Addition of `alpha` argument in plot.point function

I'd like to add a transparency (`alpha`) option in the `plot.point` function. Would you be open to a PR with this feature implementation. It will be a simple change.

## Background

I'm trying to visualize the UMAP of a very large dataset and there are some very dense clusters in the embedding space that I would like to check visually. I could raise the resolution, but it's already very big. I've found that semi-transparent dots(?) is a simple yet useful solution.

Start by locating the plot.point function in the Python plotting code and inspect how point styling is currently passed through. Check any existing plotting tests or examples available in the repository, then verify that callers can provide an alpha value and that dense plots render with adjustable transparency.

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