Contrib: Add an essentially-empty contrib dir

Context

This MR adds an essentially empty contrib directory whose purpose is to host some non-critical pieces of code. The plan is to extend the Merbocop approval rule to everything contained there in order to reduce the amount of review needed to modify non critical code. I personally plan to move Michelson Polyrun there.

Following a suggestion from @hhugo1, this MR also adds contrib to the list of directories deleted before building opam packages; this is meant to ensure that critical code cannot call anything from the contrib directory.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR