Add dune build for editor integration.

[cristianoc]

This is one way to get standard ocaml editor integration work out of the box in vscode.
At dev time one would do dune build to set up the necessary files for the editor. And repeat on file changes.

Notice this does not replace the ordinary build, which does much more than build bsc.exe.

[bobzhang]

This looks good to me since it does not interrupt existing workflow

[cristianoc]

TODO: Update build instructions:

• works on any compiler, no need for 4.06.1+rescript.
• need to do git submodule init and git submodule update if not done before
• need to do npm install and ./scripts/ninja.js config and ./scripts/ninja.js build
• Only then you can do dune build, and it's only for editor integration does not replace the normal build.
• Probably also remove mentions of buildocaml.js, mostly it would confuse people.

[ryyppy]

• Since it requires M1 users to have a built node binary for arm64, we should tell users to use node@16 (could be node 16 or above, but spares us a lot of troubleshooting when we are all on the same version)

[ryyppy]

@cristianoc pushed some changes to the CONTRIBUTING and ninja script to make things clearer