Fix compressed navigator indent highlight

[msujew]

What it does

Fixes #13114

The idea for this fix is fairly straighforward: During the rendering process, simply ignore all tree nodes that don't contribute a separate indentation to the navigator. In the compressed tree, only the head node of the compression chain actually contributes an indentation marker, so we need to check that in the new shouldRenderIndent method.

How to test

1. Create a workspace similar to what's outlined in File Explorer displays wrong indent guide with compact folders #13114
2. Assert that the navigator indent highlight is being correctly calculated

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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[martin-fleck-at]

Change looks good to me and works well, thank you very much @msujew! I just have a very nit-picky issue in one of the docs.

[martin-fleck-at]

LGTM!