feat(observable): BREAKING return disposer instead of context for chaining

[ShaMan123]

This PR makes observable easier to use by changing it's methods return values:

• return disposer for on/once
• void for fire/off

This means this PR is BREAKING.
As we discussed chainable objects are overrated and IMO are bad design because then a simple change such as this proposal becomes breaking.

So instead of:

const cb = () => {
 ...
}
obj.on('event', cb)
const dispose = () => obj.off('event', cb);
...

dispose()

This is possible

const dispose = obj.on('event', () => {
  ...
})
...

dispose()

For the dev (me for example) it saves a lot of repetitive code. It makes things easier.

[github-actions]

Code Coverage Summary

> fabric@5.1.0 coverage:report
> nyc report --reporter=lcov --reporter=text

-----------|---------|----------|---------|---------|-----------------------------------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-----------|---------|----------|---------|---------|-----------------------------------------------
All files  |   83.06 |    75.98 |   85.69 |   82.78 |                                               
 fabric.js |   83.06 |    75.98 |   85.69 |   82.78 | ...,30854,30928,30939-31004,31127,31226,31462 
-----------|---------|----------|---------|---------|-----------------------------------------------

[asturur]

More than overrated chaining, the issue is that once you opt for chaining, you can't return anything anymore, and so you cut away a bunch of comfortable patterns. Having 10% of method chainable is not really an advantage then.
We should make clear somewhere that chaining is a leftover and developers shouldn't count on it

[ShaMan123]

More than overrated chaining, the issue is that once you opt for chaining, you can't return anything anymore, and so you cut away a bunch of comfortable patterns. Having 10% of method chainable is not really an advantage then.
We should make clear somewhere that chaining is a leftover and developers shouldn't count on it

YES!!