usability: print separator between reports with dune build -w

[askvortsov1]

When I run dune build, it's pretty clear what the current errors are. The [terminal prompt] dune build line is a pretty clear separator.

Lately, I've been using dune build -w, which has a smoother integration with VSCode, and makes me do less overall. Unfortunately, this makes it difficult to see where the current error report starts, and old error reports end. I think readability would greatly improve if we printed a header at the beginning of each error report outputted by dune build -w. This could be something as detailed as

Build failed with X errors:`

(hopefully with nice colors), or as simple as a line of = or - signs.

Note that this also applies to the first error report, as dune build -w seems to clear the terminal line that invoked it.

[Alizter]

Hi, this was implemented in #6823 and will appear in Dune 3.8. There is a new command line options --display-separate-messages that can be passed.