Create duplicated XML declarations navigation

Nobody has claimed this yet.

As a developer, I want to know where are other declarations of the same XML path nodes located and access them quickly.
Area of usage is almost all XML declarations:

- di
- events
- crontab
- system
- config
- db_schema
- acl
- menu
- routes
- extension_attributes
- email_templates
- webapi
- mview
- indexer
- fieldset
- cron_group
- webrestriction
- sections

Would be nice to have something the same as we have in plugins functionality (just some icon with a dropdown list when you click on it):
List of files that contains the same declaration. If you click on it - you go directly to the selected declaration.

Example:
You have a **Vendor1_Module/etc/adminhtml/system.xml** with declaration **_section1/group2/field1_**
and **Vendor2_Module/etc/adminhtml/system.xml** with the same declaration _**section1/group2/field1**_
At both files, you have a "related nodes" icon on the left side of this node. When you click on this icon you see the list of files that also contain this declaration. Clicking to any of them you'll be switched to this file and place in file where this node is declared.

Start by reviewing the existing plugins functionality referenced in the issue and how the PHPStorm plugin handles XML declarations. Define the supported declaration areas and navigation behavior, then verify that selecting a related node opens the correct file and declaration location.

Written by the indexing model from the issue text.