fix: update docs around pluralizedTypes and type the field

[SergeAstapov]

In #902 resolver was converted to native class which has a different semantics around object initialization.

Per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields

Public instance fields are added to the instance either at construction time in the base class (before the constructor body runs), or just after super() returns in a subclass.

If you define pluralizedTypes in the app, previously the following lines run in init() hook,which runs after properties assigned to the class instance in the child class.

if (!this.pluralizedTypes.config) {
  this.pluralizedTypes.config = 'config';
}

now, above lines run before properties assigned to the class instance in the child class. Which means, pluralizedTypes defined in the child class overrides config mapping.

Because of this semantic change, any addon doing application.resolveRegistration('config:environment') would receive undefined instead of reference to the app config.

Docs should anyway teach updated syntax of pluralizedTypesdefinition.

We may argue if addons doing the right thing here or not (I would think latter is true as addons should migrate to @embroider/macros and utilize getOwnConfig, getConfig, and getGlobalConfig), but that's out of scope of this issue.

EDIT: no tests added here as there is actually nothing wrong with the addon code, it's just an app code needs to be updated.