Suggested by @Tgr in T155567#11112922

Why

• improved developer experience for user scripts and gadgets
• HTMLForm in PHP takes a data array of things on a form you want to create, and outputs HTML in ooui or codex

What

• create a MediaWiki core API called "htmlform" or similar, that will just call HTMLForm::factory() in PHP
• inputs will need to be...
  • $displayFormat - ooui, codex, etc.
  • $formItems - a data array containing all the form elements
  • $submitTextMessage - to mimic $form->setSubmitTextMsg()
• output will be HTML
• HTMLForm supports PHP callbacks. we'd need to strip those out or ignore those, since an API that just takes data and returns HTML can't use PHP callbacks
• $context - is a required param for HTMLForm::factory(). probably need to investigate exactly what parts of context HTMLForm uses, then collect that info or set it to reasonable defaults, then mock a Context object
• HTMLForm really likes using keys from the Messages API. will want to make sure to support raw text strings too, in addition to Messages API keys

Notes

• there's no MediaWiki-API tag on Phab? interesting
• example HTMLForm PHP code:

		$questionFields['options'] = [
			'label-message' => 'securepoll-create-label-questions-option',
			'type' => 'cloner',
			'required' => true,
			'create-button-message' => 'securepoll-create-label-options-add',
			'fields' => $optionFields,
			'disabled' => $isRunning,
		];

		$formItems['questions'] = [
			'label-message' => 'securepoll-create-label-questions',
			'type' => 'cloner',
			'row-legend' => 'securepoll-create-questions-row-legend',
			'create-button-message' => 'securepoll-create-label-questions-add',
			'fields' => $questionFields,
			'disabled' => $isRunning,
		];

		$form = HTMLForm::factory(
			'ooui',
			$formItems,
			$this->specialPage->getContext(),
			$this->election ? 'securepoll-edit' : 'securepoll-create'
		);

		$form->setSubmitTextMsg(
			$this->election ? 'securepoll-edit-action' : 'securepoll-create-action'
		);
		$form->prepareForm();

		$form->displayForm( $result );