Refactor to improve types for named-grid-areas-no-invalid rule

[ybiquitous]

Which issue, if any, is this issue related to?

Part of #4496

Is there anything in the PR that needs further explanation?

This change removes // @ts-nocheck from the named-grid-areas-no-invalid rule.

[jeddy3]

LGTM, thank you.

---

Incidentally, we should probably apply this rule to the grid and grid-template shorthands. I'll create an issue for it. I've been using the shorthands recently and they're great for combining areas with explicit row/column sizes, e.g.:

a {
  grid-template:
    "a a" 1fr
    "b b" 1rem
    / 1fr max-content;
}

[ybiquitous]

@jeddy3 Thanks for the review and nice suggestion about grid-template shorthand! I agree with the suggestion. 👍🏼