Add an entry type for pages in a xar extension that allows editing but also performs 3 way merge on upgrade

The current entry types supported by the platform (described here) https://extensions.xwiki.org/xwiki/bin/view/Extension/Extension%20Module/Extensions/XAR/#HStandardtypes all envisage that if a page was modified (configuration or customizable), it's never 3 way merged with updates from a newer version of the extension.

This is not fully practical for providing 'factory settings' for an extension, when applied to a page that is storing a list of configurations (such as a WebPreferences page or an XWikiPreferences page or even a custom configurations list), it is interesting to be able to propose to the user to update their instance with the new 'factory settings' of the application, even if they did customize some of these configurations for a previous version.
Notably, adding new properties to the configuration / new values is not covered by this behaviour.

Thus, this ticket is about adding a type which is similar to the customizable type but which, in addition, tries a 3-way-merge when installing a new version.

Note: this type is handled by this extension https://extensions.xwiki.org/xwiki/bin/view/Extension/Extra%20XAR%20entry%20types/ , but to me it's a core type (which should also be used for some standard pages, but this is another discussion that needs a different issue).