Add Metal shader files as source code

[ghost]

What does this PR do?

Adds support for .metal files as Metal shader source code in Xcode.

How does this PR change Premake's behavior?

Xcode projects containing .metal files will compile those files into a Metal library.

Anything else we should know?

Xcode generally puts these in the Resources category so that's what I've done, but I don't know if that's strictly necessary.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• 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)
• Follow our coding conventions
• Minimize the number of commits

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