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/Properties.json

@@ -668,7 +668,7 @@
     "inherited": true,
     "initial": "normal",
     "valid-types": [
-      "integer"
+      "number [1,1000]"
     ],
     "valid-identifiers": [
       "bold",

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);

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -290,8 +290,11 @@ public:
         return is_builtin() || is_custom_property() || is_calculated();
     }
 
+    float as_number() const;
+
     virtual String to_string() const = 0;
     virtual Length to_length() const { return {}; }
+
     virtual Color to_color(Layout::NodeWithStyle const&) const { return {}; }
 
     CSS::ValueID to_identifier() const;