add selected type to return value of showOpenFilePicker and showSaveFilePicker.

[rickg-hcl]

This issue is in response to a request in this issue:

https://bugs.chromium.org/p/chromium/issues/detail?id=1363637

The returned value should have the selected type, in addition to the list of selected files.

[a-sully]

From a spec perspective, we return sequence - the fact that it returns a JS Array that is potentially modifiable is at the bindings layer, not the spec level. We could define custom bindings, but I'm not sure how reasonable that is...

The question of "Given the current API shape, where would we put side data?" seems like a question for folks more familiar with specs. @domenic any thoughts? :)