[pyo3] Create beginGeometry/endGeometry methods in python

[catilac]

We want to create a geometry builder similar to p5js.

ref: #55 (review)

[Nitish-bot]

I'd like to work on this

[tychedelia]

Prior art:
https://p5js.org/reference/p5/beginGeometry/
https://processing.org/reference/beginShape_.html

I think we want to use the p5 flavor, where beginGeometry implicitly creates a new Geometry and endGeometry returns the id to the user.