Enable `scss/dollar-variable-default` rule (#25542)

This checks if all variables are `!default`.

.stylelintrc

@@ -221,6 +221,7 @@
     ],
     "property-no-vendor-prefix": true,
     "rule-empty-line-before": null,
+    "scss/dollar-variable-default": [true, { "ignore": "local" }],
     "selector-attribute-quotes": "always",
     "selector-list-comma-newline-after": "always",
     "selector-list-comma-newline-before": "never-multi-line",

scss/utilities/_position.scss

@@ -3,6 +3,7 @@
 // Common values
 
 // Sass list not in variables since it's not intended for customization.
+// stylelint-disable-next-line scss/dollar-variable-default
 $positions: static, relative, absolute, fixed, sticky;
 
 @each $position in $positions {