Rewrite parser and compiler in Python

Maybe it could be worth translating the [parser](https://github.com/observablehq/parser) and the [compiler](https://github.com/davidbrochart/ipyobservable/tree/49a76b4757bf769cf7d5759072a2334c7214feeb/js/lib/unofficial-observablehq-compiler) in Python, so that we process everything in the back-end before sending code to the runtime in the browser.