Add SVG backend

[nikochiko]

• Refactor and rename: backends, cairo, contexts
• Remove axes from square
• Fix bug with rendering axes
• Add comment for axes.ml
• Run dune fmt
• Default line_width to 1 for cairo
• Remove stale comment for backend_cairo.ml
• Run dune fmt
• Re-add base to joy.opam
• Add SVG backend

[FayCarsons]

Cairo has the ability to render to SVGs and PNGs simultaneously, maybe sticking to that to reduce redundancy and take advantage of those features may be a better path?

I have some other thoughts as well, will add reviews later

[nikochiko]

That won't work because we want to compile to browser. Cairo bindings won't be available in the browser.

[FayCarsons]

Are you trying to render SVG files that can be downloaded from the browser, or attach SVG elements to the DOM so they can be viewed in-browser?

[nikochiko]

Are you trying to render SVG files that can be downloaded from the browser, or attach SVG elements to the DOM so they can be viewed in-browser?

To attach elements to the DOM - so they can be viewed in browser