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/Forward.h

@@ -628,4 +628,5 @@ class ProgressEvent;
 class XMLHttpRequest;
 class XMLHttpRequestEventTarget;
 class XMLHttpRequestUpload;
+struct FormDataEntry;
 }