Improve typings in observable, computed, and map

[nettyso]

I'm very into Typescript (and I'll send you a PR of a Typescript+JSX+Snowpack Sinuous example soon!). I noticed it's hard to develop in TS with Sinuous. I'll show some examples:

Before

Here's observable and computed that both resolve to "any", so typing isn't useful:

This computed example is from your readme

Here's map showing an error about "Item":

After

The new typings infer the type given to the functions observable/computed/map which fixes these issues.

Observables won't default to Observable<any>:

Computed functions infer their return type too, so errors like this can be caught (where messages is an Observable<string[]>:

Also map doesn't use "Item" anymore, and instead infers types correctly from the function or list provided:

The only hiccup is people will see const list = observable([]) resolve as never[] if they have strict null checking on (as I do). See microsoft/TypeScript#8944. To fix this, as shown in the map picture just above, use observable([] as string[]).

[nettyso]

I put these in the order of sinuous/packages/sinuous/h/src/index.js. Felt right to include h and hs, and you hadn't updated the parameter of insert's marker to be endMark

[luwes]

thanks so much for your PR. I never really programmed in Typescript and added the typings based on other libs and after studying the TS docs a little so this is super helpful! LGTM