feat: [#1398] Add support for iframe srcdoc

[jeffwcx]

Implement the iframe srcdoc attribute, which should operate similarly to the src attribute but with higher priority, without the need to initiate a fetch request.

Also, I have fixed several bugs:

• When remove the src or srcdoc attributes, the iframe should be reloaded.
• For non-cross-domain iframes, after being connected to the Document, their contentWindow’s parent/top properties should not be null.

References:

• W3C-the-iframe-element.html#attr-iframe-src

[jeffwcx]

@capricorn86 can you take a look?

[capricorn86]

Thank you @jeffwcx for your contribution! 🌟

I've added a fix to isolate the implementation to the iframe implementation, as "srcdoc" is not really related to navigating a Browser frame.