Pixi refactor

[davidfig]

This refactor is based on the unmerged https://github.com/quadratichq/quadratic/pull/138.

• [x] replace React PIXI with vanilla PIXI
• [x] draw UI (grid lines, axes lines, headings)
• [x] draw cursor
• [x] draw multicursor
• [x] draw cells
• [x] mouse input down
• [ ] mouse input heading
• [ ] mouse input corner
• [ ] refactor keyboard input?
• [ ] plan for zoomed out caching
• [ ] clean up old files
• [ ] fix bug with fully zoomed in (either the getBounds or more likely the cull is making text disappear when there's < 1 cell per screen)
• [ ] hiding based on width/height being too small should be disabled when heading grid lines are hidden