Adds dialog form method to FormMethod enum

[Ducki]

The

element has also introduced a new form method dialog, see https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fs-formmethod

This PR adds the Dialog field to the FormMethod enum.

Fixes #44499

[ghost]

Thanks for your PR, @Ducki. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[mkArtakMSFT]

@javiercn can you please review this? Thanks!