Twenty Twenty: Vendor prefix build steps are missing configuration

[ryelle]

When building Twenty Twenty, there are two steps that run over CSS files, a vendor prefixer and RTL conversion. The vendor prefixer uses postcss, but there is no postcss.config.js file. It looks like this wasn't brought over from github when the theme was initially committed. When build is run currently, we can see an error:

You did not set any plugins, parser, or stringifier. Right now, PostCSS does nothing. Pick plugins for your case on ​https://www.postcss.parts/ and use them in postcss.config.js.

We should either remove the postcss commands from build (if so, we can also remove the dependencies), or add in a postcss config.

[ryelle]

Basic postcss + autoprefixer config in package.json