disable auto formatting for specific dune files

[Khady]

Desired Behavior

In a dune file, having a stanza (formatting disabled) would ensure that the file doesn't get touched when dune build @fmt runs.

We have dune files where the content written by hand is much more readable then when it's being auto formatted. Often because of what is explained in #10861, but not only. In those cases it would be nice to be able to preserve part of a dune file or a whole dune file.