☂️ Umbrella issue: CSS nesting and AST traversing issues

[bartveneman]

There's a couple of issues that can be resolved once we put in place an updated method of walking our AST: we need to keep track more of how rules are nested and how that influences rule sizes, complexity etc.

• Report at-rule sizes #320
• report nested @layers #288
• Calculate usage of layered vs un-layered rules #251
• Report CSS nesting #267
• Will make it possible to calculate the total complexity of any rule/atrule/declaration based on it's children