fix: ensure UNC paths are normalized correctly on Windows

[Kongkon06]

What it does

Fixes: #11997

How to test

1. Create a Windows network share
   Create a folder structure foo/bar, right-click foo → Properties → Sharing → Share… → approve UAC → finish sharing.

2. Start the Theia Electron example:
   https://github.com/eclipse-theia/theia/blob/master/doc/Developing.md#quick-start

3. In Theia, go to File → Open Folder...

4. In the navigation panel, open Network, select your machine, open the foo share, and select the bar folder.

5. Click Select Folder and verify that the UNC workspace opens and functions normally, including creating, editing, and deleting files.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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[Kongkon06]

Hi @msujew , just a friendly ping on this PR when you get a chance. It’s currently waiting on review. Thanks in advance!

[msujew]

I'll take a look at this at the start of the new year. Thank you for the patience :)