Fixes incorrect C# File Mappings for VS

[nickclark2016]

What does this PR do?

Closes #1865. Without this, no platforms were ever emitted, which is invalid in VS2022.

How does this PR change Premake's behavior?

Always emits a platform block. This is not tested in VS2019 or earlier.

Anything else we should know?

N/A

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
• [x] 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
• [x] Align documentation to your changes

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