LibWeb: Make HTMLScriptElement.src getter resolve to absolute URL

The src IDL attribute was previously implemented as an inline getter
that returned the raw attribute value. This broke spec semantics and
sites like Telegram Web that rely on document.currentScript.src to
compute Webpack’s publicPath.

According to the HTML Standard:
https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#reflecting-content-attributes-in-idl-attributes

For URL-reflecting attributes:
  1. If contentAttributeValue is null, then return the empty string.
  2. Let urlString be the result of encoding-parsing-and-serializing
     a URL given contentAttributeValue,
     relative to element’s node document.
  3. If urlString is not failure, then return urlString.

This patch moves the getter to HTMLScriptElement.cpp and implements
these steps.

Libraries/LibWeb/HTML/HTMLScriptElement.cpp

@@ -641,6 +641,31 @@ void HTMLScriptElement::unmark_as_parser_inserted(Badge<DOM::Range>)
     m_parser_document = nullptr;
 }
 
+// https://github.com/w3c/trusted-types/blob/0f01aee35c3684e8cb20e7d83445ad65938f545d/spec/index.bs#L1341
+TrustedTypes::TrustedScriptURLOrString HTMLScriptElement::src() const
+{
+    // 1. Let element be the result of running this's get the element.
+
+    // 2. Let contentAttributeValue be the result of running this's get the content attribute.
+    auto maybe_src = get_attribute(AttributeNames::src);
+
+    // 3. If contentAttributeValue is null, then return the empty string.
+    if (!maybe_src.has_value())
+        return Utf16String {};
+
+    auto const& raw = *maybe_src;
+
+    // 4. Let urlString be the result of encoding-parsing-and-serializing a URL given contentAttributeValue, relative to element's node document.
+    auto url_string = document().encoding_parse_and_serialize_url(raw);
+
+    // 5. If urlString is not failure, then return urlString.
+    if (url_string.has_value())
+        return Utf16String::from_utf8_without_validation(*url_string);
+
+    // 6. Return contentAttributeValue, converted to a scalar value string.
+    return Utf16String {};
+}
+
 // https://www.w3.org/TR/trusted-types/#the-text-idl-attribute
 WebIDL::ExceptionOr<void> HTMLScriptElement::set_text(TrustedTypes::TrustedScriptOrString text)
 {