Calculate total css complexity

[bartveneman]

We already support complexity for selectors, but could extend this to add value, property, declaration and atrule complexities. This could also clear the way for having a 'total complexity' for the whole stylesheet.

This would be awesome to graph out as well on projectwallace.com/analyze-css.

Some notes about edge cases and possible scenarios: https://astexplorer.net/#/gist/2d7b3b49ebc7094c0d0b7f29c023b757/ccf6dda1764a26a39098a90842a485916ce3fd4d

Checklist

• [x] Selectors
  • [x] Vendor prefixes
• [x] Declarations (basically whether !important is used or not)
• [x] Values
  • [x] Vendor prefixes
  • [x] Browserhacks
• [x] Properties
  • [x] Vendor prefixes
  • [x] Browserhacks
  • [x] Custom properties with and without default values
• [x] @media
  • [x] Browserhacks
• [ ] @import
  • [ ] media queries
  • [ ] layers
  • [ ] supports-queries
• [ ] @supports
  • [ ] Complexity of 'declaration'
  • [ ] Vendor prefixes
  • [x] browser hacks
• [ ] @container
  • [ ] Complexity of 'declaration'
• [x] @keyframes (whether it's vendor prefixed)