Add a multi-repl-enabled cabal to shell

[amesgen]

This adds a multi-repl-enabled (see haskell/cabal#8726) cabal (using a recent commit from master) to the shell.

When using GHC >=9.4, one can then run e.g.

cabal-multi-repl repl ouroboros-consensus ouroboros-consensus-diffusion

to get a REPL with all components in ouroboros-consensus and ouroboros-consensus-diffusion 🎉
ghcid also works, e.g.

ghcid -c 'cabal-multi-repl repl ...'

Beware that this feature is still somewhat experimental, e.g. cabal-multi-repl repl ouroboros-consensus-cardano (or also -protocol) are stalling for me, and selecting individual components involving sublibraries fails with this message:

 $ cabal-multi-repl repl ouroboros-consensus:consensus-testlib ouroboros-consensus:consensus-test
Error:
    Dependency on unbuildable library 'consensus-testlib' from ouroboros-consensus

Still, this might already be useful, and should not have any risks/maintenance burden.

[tek]

tried it out, appears to work! ran a test and changed something in the main lib and it recompiled.