Ignore the Package.swift

When you open the `Package.swift` file in VS Code you get an error:

```
[Error - 9:47:15 PM] Request textDocument/hover failed.
  Message: Could not find /Users/ryan/Source/castable-server/Package.swift in the workspace.
  Code: -32099
```

This is because the file is not in the "project" as defined by the language server.