feat(explicitEffect): addition of defer parameter; this parameter all…

[mauriziocescon]

As part of the discussion related to explicit deps tracking for the effect function, @JeanMeche delivered the implementation of explicitEffect. 🎉

This is a small PR that complements the initial implementation by adding an optional defer parameter in order to skip the initial callback call (by design effects run once).

Here is a scenario describing its usage: angular/angular#56155 (comment)

Note that the other frameworks have a similar functionality:

• solid (on + defer): https://www.solidjs.com/tutorial/reactivity_on,
• vue (watch, lazy by default): https://vuejs.org/guide/essentials/watchers#eager-watchers,
• MobX (reaction): https://mobx.js.org/reactions.html#reaction

As part of the PR, I've also reworked a bit the doc (hope you don't mind).

[JeanMeche]

With the upcomming changes to effect, I not sure this is a good idea.

See angular/angular#56501, we'll have root effects and component effects.

[mauriziocescon]

Umh... noticed the PR, but I'm not sure I get all the details (probably I'll wait for your blog-post 😅).

In few words: same effect function with different behaviours depending on component and (for example) root service? I was anyway assuming the nature of effects (always run at least once) was unchanged (defer = skip first callback call)...