linter: Lint grid-area names for quotation marks.

This adds a CSS stylelint rule that will catch erroneous quotation
marks around named grid-areas, which should be <custom-ident> values,
not strings.

So, for example, `grid-area: "my_area";` is an error. It should be
`grid-area: my_area;`.

stylelint.config.js

@@ -28,6 +28,9 @@ module.exports = {
             "/^(border(-top|-right|-bottom|-left)?|outline)(-width)?$/": [
                 /\b(thin|medium|thick)\b/,
             ],
+            // no quotation marks around grid-area; use
+            // `grid-area: my_area`, not `grid-area: "my_area"`
+            "grid-area": [/".*"/],
         },
         "function-disallowed-list": [
             // We use hsl instead of rgb