LibWeb: Fix up constructing the form entry list

In particular:
- Don't include none submitter buttons.
- Use type_state() instead type() to avoid direct string comparisons
- Support the hidden _charset_ input
- Get form associated element's value directly instead of via the value
  attribute
- Split line break normalization into a separate function so that it
  can also be used by form submission.

Userland/Libraries/LibWeb/HTML/HTMLButtonElement.cpp

@@ -99,4 +99,20 @@ i32 HTMLButtonElement::default_tab_index_value() const
     return 0;
 }
 
+// https://html.spec.whatwg.org/multipage/forms.html#concept-submit-button
+// https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element:concept-submit-button
+bool HTMLButtonElement::is_submit_button() const
+{
+    // If the type attribute is in the Submit Button state, the element is specifically a submit button.
+    return type_state() == TypeAttributeState::Submit;
+}
+
+// https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element:concept-fe-value
+DeprecatedString HTMLButtonElement::value() const
+{
+    if (!has_attribute(AttributeNames::value))
+        return DeprecatedString::empty();
+    return attribute(AttributeNames::value);
+}
+
 }