chore: migrate to upstream sax

[SethFalco]

Migrates from our maintained unmaintained fork of sax-js to upstream.

While sax-js is not actively maintained, it has received some updates which has allowed it to catch up with our fork, and has more bug fixes in it. I've also made contributions to sax-js myself to get here.

• fix: ignore unparsed entity logic for predefined xml entities isaacs/sax-js#266
• fix: don't throw on comment in dtd isaacs/sax-js#267
• feat: add explicit option for unquoted attribute values isaacs/sax-js#268
• fix: doctype with entities handling isaacs/sax-js#272

There is one change we needed that was not merged upstream, but instead we apply a patch via Yarn in this repository. This is acceptable in this case as the change only impacts the browser bundle, which is our job to generate, and contains the change to lib/sax.js in the following commit:

• svg/sax@ef73722

Related

• Fixes Error in parsing SVG: Unexpected end #879
• Closes refactor svg2js to use txml #1301 (supersedes)
• Support unquoted attributes #678 - Sax has a new option which allows us to support unquoted attributes while in strict mode. I'm just not certain if we want to use this option, or if we should continue to throw errors in this case. Feedback welcome!