Named form getter
This PR contains changes related to adding named getter in Servo for getting the list of all meaningful property names for a HTMLFormElement object, and getting the value of a specific property name. The following changes have been made:
* uncomment the [named getter](f63b404e0c/components/script/dom/webidls/HTMLFormElement.webidl (L30)) from HTMLFormElement.webidl
* add the missing `NamedGetter` and `SupportedPropertyNames` methods to [HTMLFormElement](f63b404e0c/components/script/dom/htmlformelement.rs (L113))
* implement `SupportedPropertyNames` according to [the specification](https://html.spec.whatwg.org/multipage/forms.html#the-form-element:supported-property-names):
* create an enum to represent the `id`, `name`, and `past` states for the sourced names
* create a vector of `(SourcedName, DomRoot<HTMLElement>)` by iterating over `self.controls` and checking the element type and calling methods like `HTMLElement::is_listed_element`
---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#16479 (GitHub issue number if applicable)
d205194618