add bisect_ppx support

[zapashcanon]

The idea would be to have a boolean flag in projects (which can be overridden on a per package basis) to enable bisect_ppx support.

If it is enabled, we should add something like:

(instrumentation (backend bisect_ppx))

to all dune stanzas of the packages.

Once this is done, we can add some code to the deploy-doc GitHub action to run the coverage and put the result in doc/coverage and add a link in the index.html page of the documentation.

I did it by hand in toml to see what it would look like. See :

• homepage with the link to coverage results added
• coverage report generated
• GitHub action code used to generate it