Generate a sourcemap

I have a use case for tracking old and new string indexes of the before + after CSS (CSS Coverage inspector which needs to keep track which chunks of CSS are covered before/after prettification of the CSS).

This could also be done via a callback, like this, but sourcemaps are more elegant, I guess?

let positions = new Map<[number, number], [number,number]>
let new_css = fomat(old_css, { 
  on_node: function (node: csstree.CssNode, start_pos: number, end_pos: number }) {
    positions.set([node.loc.start.offset, node.loc.end.offset], [start_pos, end_pos])
  })
})

The issue names no files, tests, or entry points. Start by locating the TypeScript formatting pipeline and reviewing how source positions are represented. Done means exposing a sourcemap that relates old CSS string indexes or node ranges to their positions in the formatted CSS.