Analyzer to suggest using `readonly struct`

[SteveDunn]

It is preferable for users to use a readonly struct rather than just a struct. Implement an analyzer to suggest this, and possibly a codefix.

This was a result of the following thread.

          @SteveDunn A suggestion that came to my mind while reading this: maybe enhance the analyzers to promote the usage of `readonly struct` instead of a `struct` as an _Information_ or _Warning_ message?

Originally posted by @Herdo in https://github.com/SteveDunn/Vogen/issues/52#issuecomment-1230748733