improve flow typings for observable

[halbgut]

This is basically just copied from the typescript definition and slightly modified to match the flow syntax.

[coveralls]

Coverage remained the same at 94.651% when pulling f43dbf6 on Kriegslustig:feature/improve-flow-typings-for-observable into b7fef45 on mobxjs:master.

[halbgut]

CI failure because of auto cancellation. Can't really do anything about this. The tests pass locally.

This job was cancelled because the "Auto Cancellation" feature is currently enabled, and a more recent build (#1232) for pull request #1231 came in while this job was waiting to be processed.

[coveralls]

Coverage remained the same at 94.686% when pulling bb47f40 on Kriegslustig:feature/improve-flow-typings-for-observable into de51957 on mobxjs:master.

[mweststrate]

Thanks a lot for your effort!