Add workspace_root variable expansion in paths field

[bobot]

follow-up of #2476. It gives a way to reference the source directory by using %{workspace_root}.

[rgrinberg]

Could you also support project_root? Actually using workspace_root should really be discouraged as it makes your dune build non-composable. I'm on the fence on whether we should be adding more support for it.

[nojb]

Could you also support project_root? Actually using workspace_root should really be discouraged as it makes your dune build non-composable. I'm on the fence on whether we should be adding more support for it.

True; but (paths) is only supported on dune-workspace for now, so it wouldn't make sense to use project_root, right? Maybe we should wait until there is support for (paths) in dune files and only do the expansion there?

[ghost]

Looks good!

[rgrinberg]

@bobot closing as this stale. feel free to re-open if you rebase.