[vs2010+] Handle `compileas` for files with "unknown" extensions.

[Jarod42]

What does this PR do?

Handle compileas for files with "unknown"/mismatching_categories extensions for vs* generator.

How does this PR change Premake's behavior?

Only vs* generators impacted.

Anything else we should know?

Tested with https://github.com/Jarod42/premake-sample-projects/tree/vs_compileas/projects/project-compileas with action: https://github.com/Jarod42/premake-sample-projects/actions/runs/8392851053

Did you check all the boxes?

• [x] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [x] Add unit tests showing fix or feature works; all tests pass
• [ ] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [x] Follow our coding conventions
• [x] Minimize the number of commits
• [ ] Align documentation to your changes