[css-mixins-1] Let 'inherit' work like 'inherit()'

[andruud]

With @function, it becomes possible to have a "type mismatch" between an "outer" custom property and an "inner" custom property:

@property --x {
  syntax: "auto | red";
  /* ... */
}

@function --f(--x <color>: inherit) {
  result: var(--x);
}

div {
  --x: red;
  --result: --f(); /* => ? */
}

We have to address this somehow, and the obvious way seems to be making 'inherit' behave like "a var(--x) that resolves in the parent stack frame". This is essentially what inherit() does, which already exists in the spec.