Implement onstate and onupdate hooks

[Rich-Harris]

Still to do: add deprecation warnings when this.observe is called, and add this.observe to svelte-extras (which can't happen until this is released, as it will depend on the state and update events).

An unanticipated effect, which I think is actually a positive, is that we need store.on and store.fire. So apps now get a global event bus for free. (store.onchange also needs to be deprecated.)

It might also be possible to replace _bind with component.on('state'), which would be nice.

[codecov-io]

Codecov Report

Merging #1344 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   91.76%   91.78%   +0.02%     
==========================================
  Files         125      127       +2     
  Lines        4515     4531      +16     
  Branches     1432     1443      +11     
==========================================
+ Hits         4143     4159      +16     
  Misses        149      149              
  Partials      223      223

Impacted Files	Coverage Δ
src/generators/nodes/Component.ts	96.9% <ø> (ø)	⬆️
src/validate/js/propValidators/index.ts	100% <ø> (ø)	⬆️
src/generators/dom/index.ts	95.9% <100%> (+0.14%)	⬆️
src/generators/Generator.ts	96.71% <100%> (+0.03%)	⬆️
src/validate/js/propValidators/onupdate.ts	100% <100%> (ø)
src/validate/js/propValidators/onstate.ts	100% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8a99ce9...3d8c768. Read the comment docs.