…tems when query changes

Summary:
Address issue microsoft#73904 by adding an optional `sortByLabel` to the QuickPick class which determines whether the picker re-sorts the result list when the user types in the input field.

If true, the picker applies a sort to order results by the index of the first appearance of the input in the label.

For backwards compatibility, this field is true by default.

microsoft#73904

Test Plan:
attached video shows behavior both before and after

{F167292605}

note: there aren't any existing tests on what happens when the query input changes in the QuickPick

Reviewers: dalongi, ericblue, hchau

Reviewed By: ericblue

Differential Revision: https://phabricator.intern.facebook.com/D16203434

Signature: 16203434:1562878837:5413e3852f2bd04c8e81b9fe5c4a08127dfe3b65

Summary: API additions should start off in the proposed API file.

Test Plan:
same as before, tested that I could skip sorting:
{F168164535}

Reviewers: dalongi, ericblue, hchau

Reviewed By: ericblue

Differential Revision: https://phabricator.intern.facebook.com/D16264910

Signature: 16264910:1563214616:55ecf86a9d4a0d8a0cd44d9b33eb770b2a7d04aa