This adds a new IDL type, Utf16DOMString. This is the same as DOMString,
except it is UTF-16. This type is temporary - we will want DOMString to
be UTF-16 by default once we've ported enough of LibWeb.
To make this support easier, some string IDL generator handling is moved
directly into `generate_to_string` from the call sites.
Partial interfaces have the same name as the interface they extend, and
can appear in any order. In practice, we import the partial interfaces
into the main interface's IDL.
This adds support for async iterators of the form:
async iterable<value_type>;
async iterable<value_type>(/* arguments... */);
It does not yet support the value pairs of the form:
async iterable<key_type, value_type>;
async iterable<key_type, value_type>(/* arguments... */);
Async iterators have an optional `return` data property. There's not a
particularly good way to know what interfaces implement this property.
So this adds a new extended attribute, DefinesAsyncIteratorReturn, which
interfaces can use to declare their support.
This fixes the frame-ancestors WPT tests from crashing when an iframe
is blocked from loading. This is because it would get an undefined
location.href from the cross-origin iframe, which causes a crash as it
expects it to be there.
