Fix "escaped newline symbol" bug in CodeGeneration module

[RoBo-Inc]

I stumbled upon a small bug by accident, and upon investigation, it appeared to have deep roots. Eventually, it warranted its own PR. The bug involved duplicating empty lines in the generated SyntaxTraits file after every // MARK: - without documentation text.

The file is generated based on SyntaxTraitsFile file of CodeGeneration module:

      // MARK: - \(trait.protocolName)

      \(trait.documentation)
      public protocol \(trait.protocolName): SyntaxProtocol\(raw:

In other parts of the module, similar situations are addressed using the "escaped newline" symbol: "". For instance, in the ResultBuilderFile file:

      // MARK: - \(type.resultBuilderType)

      @resultBuilder
      \(node.node.apiAttributes())\
      public struct \(type.resultBuilderType): ListBuilder

So in this PR, I resolve the bug by employing the same approach. This not only enhances the consistency of the codebase but also serves as an elegant solution to the problem itself.
All the changes are confined to the CodeGeneration module and have no impact on the rest of the SwiftSyntax library, making them safe to merge.

[ahoppen]

Nice. Much simpler

[ahoppen]

@swift-ci Please test