Fix playground infra for ReScript / dune setup

[ryyppy]

ReScript 11 has switched over to a dune centric workflow, therefore we need to migrate some stuff from ninja.js and repl.js over to dune and make.

Mostly the following things need to be fixed:

• Update the cmij file generation to also generate the ReScript lib/ocaml modules as a compilerCmij.js file
• Update the CONTRIBUTING file to reflect the latest development workflows
• Wire everything up in our Makefile so it's easy to understand how to build and release a new playground bundle

Extra work:

• Add @rescript/core as a pre-vendored dependency for the released playground files
• Make the playground and third party cmijs files use the uncurried mode by default