LibWeb: Adapt parse_position() into parse_position_value()

Having two ways that `<position>` is represented is awkward and
unnecessary. So, let's combine the two paths together. This first step
copies and modifies the `parse_position()` code to produce a
`PositionStyleValue`.

Apart from returning a StyleValue, this also makes use of automatic enum
parsing instead of manually comparing identifier strings.

Userland/Libraries/LibWeb/CSS/Enums.json

@@ -334,6 +334,7 @@
     "sticky"
   ],
   "position-edge": [
+    "center",
     "left",
     "right",
     "top",