Use form.element.namedItem to track associated inputs.

[dmarkow]

Closes #4102. A couple tests are failing, but they were already failing when I first cloned the repo.

Before, it was required that all form elements be nested under the form. However, forms are allowed to have inputs located elsewhere as long as they use the form="my-form-id" attribute.

This changes it to use form.elements.namedItem(name). form.elements is already used in resetForm so this change is consistent with existing code.

Note that radio/checkboxes with the same names will return a list, whereas querySelector only returned the first result, so I'm only matching on the first element to match existing behavior. I'm not familiar enough with what exactly incCycle/deletePrivate are doing, but I can update this to run those functions on all matching elements if needed.

[SteffenDE]

Thank you!