Refactor Indiekit class

[paulrobertlloyd]

• Have plug-ins register features directly onto properties of Indiekit class
• Use Set() for Indiekit.endpoints, Indiekit.installedPlugins, Indiekit.locales and Indiekit.stores
• Use Map() for Indiekit.localeCatalog, Indiekit.postTypes and Indiekit.validationSchemas (in addition to preceding refactor which introduced a map for Indiekit.collections)
• Rename bootstrap to updatePublicationConfig; now that this method has a single purpose we can give it a clearer name
• The default config now shares the same shape as that used by consumers and no longer adds private or undocumented values.

Introduces the following breaking API change:

• Indiekit.config.application.postTypes object replaced by Indiekit.postTypes map

While this makes the Indiekit class and its properties and methods clearer, some confusion is still potentially created in that these properties are added to request.app.locals under the application key. Refactoring how locals are named and assigned, and the impacting changes to endpoints that use these values will be tackled in a separate PR. Ideally by the end of this refactor, it should be clearer how Indiekit is structured.