feat: allow customizing the point size's zoom scale function

[flekschas]

This PR introduces a new property called size_scale_function to allow customizing the point size's zoom scale function.

Description

What was changed in this pull request?

This PR exposes regl-scatterplot's pointScaleMode (added by @abast! Thanks again 🎉 ) via scatter.size(scale_function). You can choose between

• asinh (default)
• linear
• constant (a.k.a., no zoom-based scaling)

Why is it necessary?

Because it's a neat feature to have and it's already implemented in regl-scatterplot

Demo

Zoom-based scale functions in the following order:

1. asinh
2. linear
3. constant

asing-linear-constant.mp4

Checklist

• Provided a concise title as a semantic commit message (e.g. "fix: correctly handle undefined properties")
• CHANGELOG.md updated
• Tests added or updated
• Documentation in API.md/README.md added or updated
• Example(s) added or updated
• Screenshot, gif, or video attached for visual changes

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