LibWeb: Implement the CSS outline property :^)

...along with `outline-color`, `outline-style`, and `outline-width`.

This re-uses the existing border-painting code, which seems to work well
enough!

This replaces the previous code for drawing focus-outlines, with generic
outline painting for any elements that want it. Focus outlines are now
instead supported by this code in Default.css:

```css
:focus-visible {
    outline: auto;
}
```

Userland/Libraries/LibWeb/CSS/Enums.json

@@ -245,6 +245,18 @@
     "none",
     "scale-down"
   ],
+  "outline-style": [
+    "auto",
+    "none",
+    "dotted",
+    "dashed",
+    "solid",
+    "double",
+    "groove",
+    "ridge",
+    "inset",
+    "outset"
+  ],
   "overflow": [
     "auto",
     "clip",