LibWeb: Add range-checking to property_accepts_value()

For `number` and `integer` types, you can add a range afterwards to add
a range check, using similar syntax to that used in the CSS specs. For
example:

```json
"font-weight": {
  ...
  "valid-types": [
    "number [1,1000]"
  ],
  ...
}
```

This limits any numbers to the range `1 <= n <= 1000`.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -98,6 +98,12 @@ bool StyleValue::is_color() const
     return false;
 }
 
+float StyleValue::as_number() const
+{
+    VERIFY(is_numeric());
+    return static_cast<NumericStyleValue const&>(*this).value();
+}
+
 String IdentifierStyleValue::to_string() const
 {
     return CSS::string_from_value_id(m_id);