[FEAT] Add fpe-observes codemod

[rajasegar]

#21

Function.prototype.observes

Historically, Ember has extended the Function.prototype with a few functions (on, observes, property), over time we have moved away from using these prototype extended functions in favor of using the official ES modules based API.

Before

import EmberObject from '@ember/object';

export default EmberObject.extend({
  valueObserver: function() {
    // Executes whenever the "value" property changes
  }.observes('value')
});

After

import EmberObject, { observer } from '@ember/object';

export default EmberObject.extend({
  valueObserver: observer('value', function() {
    // Executes whenever the "value" property changes
  })
});