feat: add rxjs operators `poll` and `whenDocumentVisible`

[FabienDehopre]

Add two RxJs operators that works well together but still can be used independently.

poll can be used to poll an API at a fixed interval with an optional initial delay. whenDocumentVisible is the RxJs counterpart of the injectDocumentVisibility and can be used to pause and resume a RxJs stream based on the document visibility state.

Related to #405

[eneajaho]

PR not found

[FabienDehopre]

What do you mean?

[nartc]

@all-contributors please add @FabienDehopre for code

[nartc]

@all-contributors please add @FabienDehopre for code

[nartc]

@FabienDehopre looks like the test failed. Can you take a look?

[nartc]

@all-contributors please add @FabienDehopre for code

(third time the charm?)

[allcontributors[bot]]

@nartc

I've put up a pull request to add @FabienDehopre! :tada:

[nartc]

@all-contributors please add @FabienDehopre for code

(fourth time 🥲)

[allcontributors[bot]]

@nartc

@FabienDehopre already contributed before to code

[FabienDehopre]

I've fixed the failing tests.