Add option to generate type-per-line

For syntax highlighting purposes in a diff I want to know what sort of thing we're rendering in each line. So apart from returning the CSS as a string I also want an array if types:

import { format_with_types ] from '@projectwallace/format-css'

let { css, types } = format_with_types('a { color: red; }')

// => css
`a {
  color: red;
}`

// => types
[
  1, // selector
  2, // declaration
  3, // bracket
]

The following types are needed:

• selector (a)
• declaration (color: red;)
• bracket ({, })
• atrule (@media all)

This is sufficient for highlighting purposes because the formatting rules dictate that every atrule or selector prelude line end with a { and every declaration ends with a ;.

We don't need to support this for minified CSS because that's just madness.

The issue names no implementation files or tests. Start by reading the formatter API and the linked formatting rules, then trace how formatted lines are produced; done means non-minified output exposes the CSS and a matching per-line array of selector, declaration, bracket, or atrule types.