📎 CSS Parser: Parse all and z-index properties

[faultyserver]

Description

#268. These will be the first properties that we parse exactly, meaning we know the structure that the property value is supposed to have and parse that into named fields directly in the syntax tree.

Since this is the first, it will also involve setting up the system for dispatching parsing for each property, and establish the structure for how the grammar gets defined and what parsing looks like.

---

https://drafts.csswg.org/css-cascade/#all-shorthand
all =
  <global-keyword>

https://drafts.csswg.org/css2/#z-index
z-index =
  auto |
  <integer> |
  inherit