Markdown pages

Adds support for Markdown (and MDX) pages as first-class citizens.

Note: You will need to update your page loader in our apps (apps/YOUR_APP/src/common/page-loader.ts) in order to take advantage of this functionality. i.e:

export const pageLoader = require.context('./pages', true, /\.([jt]sx?|mdx?|html)$/i, 'sync');

(.md and .mdx are now covered by the regular expression.)

Also, SCSS builds should now be a bit faster, thanks to the use of sass-embedded.

BREAKING:

• Node.js v16 is no longer officially supported and may break now or at some point in the future

Misc:

• sass-loader updated to v16 (should not be breaking)
• css-loader updated to v7 (should not be breaking)
• sass replaced by sass-embedded (should not be breaking)