Dune: mark vendors/ directory with vendored_dirs

This prevents dune from recursing into the vendors/ directory when building all targets, running all tests, or linting with dune build @fmt. Libraries in the vendors/ directory are also built with warnings suppressed.

This has the effect of avoiding running tests for ocaml-blake2, ocaml-lmdb etc. as part of the make test command.

See https://dune.readthedocs.io/en/stable/dune-files.html#vendored-dirs-since-1-11 for more details.