Expose OutliningSpans as official TSServer API?

[mjbvz]

@aeschli is looking into language aware folding in vscode this iteration and we'd like to use theOutliningSpans TSServer API to implement this, however it is currently internal: https://github.com/Microsoft/TypeScript/blob/70e9a5e6dc02e5eb01e8dac3e0235a096b4d232f/src/server/protocol.ts#L91

Is this API ready for external usage?