LibWeb: Implement `HTMLFormElement.requestSubmit()`

[tcl3]

This PR implements the HTMLFormElement.requestSubmit() method, which can be used to programatically submit a form using a specific submit button.

My motivation for adding this is that I wanted to use it in a test I had previously written; I've taken the opportunity to update that test as part of this PR.

This method also appears to be used by some WPT tests.