Correctly duplicate files with numbers in filename

[imwiss]

This PR fixes an issue with duplicating files with numbers in the filename. It now correctly duplicates files with numbers in filename by appending a version instead of incrementing the integer.

Before:
Duplication of "1.js" gives "2.js".

Now:
Duplication of "1.js" gives "1.1.js"

Original issue: #22876

[mention-bot]

@wabirached, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bpasero and @egamma to be potential reviewers.

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[bpasero]

@wabirached a filename called test.35.txt still ends up as test.36.txt when I copy it, can we improve it for that case as well?

[imwiss]

hey @bpasero, I thought that was the intended behaviour based on this comment: https://github.com/Microsoft/vscode/pull/22907/files#diff-9404b1da682862c61dce10b84ff76e5cR1101

This makes sense if a user is duplicating the file multiple times (test.txt -> test.1.txt -> test.2.txt -> etc).

It's also the way it worked in previous builds (and it's consistent with how other products behave). Let me know what you think.

[bpasero]

@wabirached of course, good point, I oversaw this.