Complete PR and all review items.

I've produced https://www.github.com/jacomyal/sigma.js/pull/1252. Today I'm starting to revise it until there's nothing extra or unnecessary. The original PR was based on an experimental research branch that produced the fruit I was looking for. Now I want that PR to meet all of your expectations. I'm going to keep a checklist here for now, tracking my progress as I update the PR.

• Get rid of the .glsl shaders in the demo.

I moved these shaders from the sigma.js hierarchy because I was having trouble importing them. I'll spend time making that work as expected.

• Remove generate-graph.ts

Just gonna go with .js on this one.

• Add arguments and messaging to generate-graph.js

• Modify index.html to display graph parameters instead of take them.

• Consider a button to switch between GPU and CPU data ETL with benchmark.

The above is not going to happen in this PR, it'll take another PR if we get there. I'll definitely be looking at more benchmarks if there's interest.

• Clean up index.ts

• Document the hacks in rapids-graph.ts and the engine improvements they imply.

• Modify package.json to handle launching on a different port specifically for extra-large-graphs

• Reset and minimize changes outside of extra-large-graphs.

• Make a sweet README.md about what is going on here.

Depends on #1252