feat(bindable): support getter/setter

[bigopon]

📖 Description

resolves #1602

Currently, bindable can only be used with a field, which means the following usages are invalid:

An attribute with a readonly bindable

@customAttribute('my-attr')
class MyAttr {
  private _m = [{ v: 'hello' }, { v: 'world' }];

  @bindable({ mode: BindingMode.fromView })
  get message() {
    return this._m.map(v => v.v).join(' ');
  }
  ...
}

And more similar logics, where a pair of setter/getter are more desirable than a field.

This PR refactors the bindable infra, as well as adds support for the above usage. It does so by treating bindable just as bindable metada for compilation, and enhance existing observers with capabilities to perform work of the bindable observation.
With this work in place, supporting the new decorator proposal will be much easier since we are capable of handle getter/setter from the new auto accessor feature.

📑 Test Plan

• all existing tests should work
• add new tests for bindable with getters/setters, readonly getter/ coercion + getter/setter

cc @brandonseydel