Allow optional SCSS support or custom PostCSS plugins

[romannurik]

I most miss the ability to nest rules... It'd be nice to either support modules like postcss-nested (or whatever is defined in postcss.config.js) or SCSS support as an option.

[romannurik]

I think I found a hidden configuration option:

1. Create a file named build-figma-plugin.ui.js
2. Put the following contents in the file (and install esbuild-sass-plugin):

const { sassPlugin } = require('esbuild-sass-plugin');

module.exports = config => {
  config.plugins.push(sassPlugin());
  return config;
};