Add SVG encoder

[patriksvensson]

This PR is mainly for discussion, so let's not merge it yet.

I've pretty much ported all the code for this PR from Rich: Textualize/rich#2101, so if we decide to merge it, we should make sure to add attribution.

• Is this the road we want to take with SVG rendering, or should we try to build something similar to what @phil-scott-78 did with the Asciicast encoder?
• Should this be in a separate repo, away from the core project?

Todo

• Add proper attribution to Rich
• Un-internalize the theme classes and the settings

Example