Preserve comments in output _pb.jl files from original .proto

[NHDaly]

We should ideally copy all the comments from the original .proto file into the new generated _pb.jl file by reading the comments fields off of the SourceCodeInfo proto.

An example that does this in Go can be found here (under the MIT license):
https://github.com/pseudomuto/protokit/blob/7037620/comments.go

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This issue is extracted from #128, which was never merged, and is obsolete after the 1.0.0 rewrite.