frint-store: interop with all reactive libraries

[fahad19]

Note: NOT a breaking change

What's done

frint-store now supports interoperability with other reactive libraries, following the TC39 proposal here: https://github.com/tc39/proposal-observable#observablefrom

Usage

As of latest release, we can get our state stream as an Observable like this:

const state$ = store.getState$();

With this PR, we now adhere to common standards as per the TC39 proposal, and can get the state stream by doing:

const state$ = Observable.from(store);

The store.getState$() still remains in the API, and uses from(store) internally.

Why

This will enable us to build more generic tools without having to know too much about the internal APIs of frint-store.

More will follow in #391

[codecov]

Codecov Report

Merging #393 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   97.33%   97.34%   +<.01%     
==========================================
  Files         101      101              
  Lines        3908     3918      +10     
==========================================
+ Hits         3804     3814      +10     
  Misses        104      104

Impacted Files	Coverage Δ
packages/frint-store/src/Store.js	100% <100%> (ø)	⬆️

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