Add `SplitByType`s observables

[HollandDM]

As discussed in #103, this is the implementation of SplitByType macro for observable. The macro will expand into match/case definition, so it can takes advantage of exhaustive and reachable checking of the compiler.

[ngbinh]

@kitlangton @raquo is there anything left to do with this PR? I think it is quite useful

[raquo]

It's definitely very useful, thanks for your work on this! I am yet to properly review this, but on the high level it looks good.

Sorry for such a delay, this was just too much for me to include in v17. I plan to merge this when I start working on the next Laminar release, likely around September-October. Since all this ties into Airstream with extension methods, I assume that you can use this code privately for now.

[ngbinh]

Awesome! Thanks @raquo