This adds parsing support for `partial interface mixin` as specified
in WebIDL. Partial mixins are merged into their corresponding mixins
before the mixins are resolved into interfaces.
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 allows us to specify multiple base paths to look for imported IDL
files in. This will allow us to import IDL files from sources and from
the Build directory (i.e. for generated IDL files).
