Implement "the rules for choosing a browsing context"

[gterzian]

Take 'target' into account when following the hyperlink (and perhaps also other small changes, it seems the labeling of some steps needs updating and others seems to be missing).

Loading the url in a target in a different browsing context seems to be blocked by #13241(but I'm wondering if that should only be if it's in a new 'top level browsing context'? Can we still follow the hyperlink in a iframe of the current page, like is necessary for the test below?).

Spec: https://html.spec.whatwg.org/multipage/links.html#following-hyperlinks-2

Code:

servo/components/script/dom/htmlanchorelement.rs

Line 583 in 05fe8fa

pub fn follow_hyperlink(subject: &Element, hyperlink_suffix: Option<String>, referrer_policy: Option<ReferrerPolicy>) {

test: https://github.com/servo/servo/blob/05fe8fa08d507836ce5659ff56f83022a90b241a/tests/wpt/web-platform-tests/html/browsers/browsing-the-web/navigating-across-documents/007.html

[gterzian]

On second thoughts, I think this issue is more about implementing https://html.spec.whatwg.org/multipage/browsers.html#the-rules-for-choosing-a-browsing-context-given-a-browsing-context-name, which is useful in other places besides the anchorelement, such as forms, see

servo/components/script/dom/htmlformelement.rs

Line 340 in 05fe8fa

let _target = submitter.target();

so I propose to do that, and then plug it both into forms and anchors.

[gterzian]

Would such a choose_browsing_context method belong perhaps on WindowProxy?