Summary:
LSP `CompletionList` supports an `isIncomplete` property.

when false (the default), the IDE triggers a completion request when you type a trigger character and filters locally as you keep typing, without making further requests.

when true, the editor continues sending completion requests as the user types the same token, allowing the server to do its own filtering (useful when there are too many results).

we currently always set `false`, but this change sets up a later change.

Reviewed By: vrama628

Differential Revision: D23962030

fbshipit-source-id: 90eace31ebab197117c34102e26edaca97ba411d