plugin-ext: default the folderExpanded icon

[cdamus]

What it does

In file icon themes, VS Code will use the "folder" icon also for expanded folders in the case that the theme omits the "folderExpanded" entry.

This PR implements the same fall-back in Theia for consistency.

Fixes #12727

How to test

1. Follow the OP's directions in Unexpected folder behavior with file icon theme contribution from VS Code extension #12727 to clone and build a same myext extension with a custom file icon theme.
2. Link the myext extension package into your plugins/ folder.
3. As directed in the issue description, launch Theia and select the "My File Icon Theme" file icon theme.
4. See that the custom folder icon is shown in both expanded and collapsed states.

Furthermore, edit the file icon theme in the myext to

• set a different icon (the extension resources/ conveniently includes additional icons) as the "folderExpanded" and see that it works as expected, showing that icon in expanded state and the other in collapsed state
• set "folderExpanded": null or "folderExpanded": "" to explicitly ask for no icon for expanded state and see that this also works

Review checklist

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

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍

[cdamus]

Thanks, @msujew commit 54b363a implements a for TS-ish test expression.

[jcortell68]

@cdamus Thanks for fixing this!