Show Assembly name in Unity Explorer for current folder or file

Pull request #989 changed how the Unity Explorer looks. A related issue is #982
Before, it was very noisy with assembly names being shown on each folder.

I'm currently working on a large project and I often don't know what assembly I'm in when I'm jumping around some Type's reference chain.

I think it would be nice to have an option to show assembly info the old, "noisy", way, for the currently selected file (or at least for the currently selected file's parent folder).
(It would actually be even nicer to have for both the currently selected file's folder AND the file itself, since sometimes you have too many files in a folder to actually be able to see the folder without scrolling.)

Since we now also have .asmref files, it's sometimes quite hard to know in what assembly you're in, exactly.

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In case it is not commonly known, asmref are "assembly definition reference files". Those allow you to say a folder belongs to a different assembly. For example, it can be used to group all of your "Editor" folders a single "YourAssembly-Editor" assembly, without having to modify your folder structure.
This feature is particularly useful for asset store packages that you want in a custom assembly, but don't want to modify its structure.

I think it was first introduced in [Unity 2019.2.0a6](https://unity3d.com/unity/alpha/2019.2.0a6).

Start by reviewing pull request #989 and related issue #982 to understand the current Unity Explorer behavior and how the selected file or folder is represented. Trace the existing assembly-name display logic, then define the option's behavior for the selected file, its parent folder, and .asmref cases; done means the chosen context visibly identifies its assembly without restoring noise everywhere.

Written by the indexing model from the issue text.