Support for esy local development.

[jordwalke]

Summary: This diff adds support for local development of the graphQL ppx
via esy. I added instructions about usage in the new DEVELOPING.md doc
and I hope some more info can be added there about how the CI system
works etc.

This doesn't implement all the CI and packaging using esy which will
eventually become possible once we get deep Windows support, but it
makes the local install/build iteration more enjoyable.

I also moved to the officially supported Dune approach for configuring
builds (discover.ml) which is what the Dune devs recommend. It also
was required for the local esy builds because esy doesn't want you to
write artifacts to your source tree (when your esy.json file
says it only writes to the _build directory).

Test Plan:Tried this out with esy and it works.

[rgrinberg]

Don't do this as you will break cross compilation. Use ocaml_config_var_exn for such environment tests. See my example here: https://github.com/savonet/ocaml-ssl/pull/39/files#diff-81f275ec1ff7428a19d7cd928fb69c94

Also, use the configurator entry point function: C.main.

[mhallin]

It looks like Esy and npm/yarn are fighting against each other about what should be in the node_modules folder:

$ esy install
...
info installing: done

$ esy build
# Completes successfully 

$ esy test
dune build @graphql_ppx
cp _build/default/src/graphql_ppx.exe .
(cd tests && \
		../node_modules/.bin/bsb -clean-world && \
		../node_modules/.bin/bsb -make-world)
/bin/sh: ../node_modules/.bin/bsb: No such file or directory
make: *** [test] Error 127

# Fails, as expected - the node dependencies have not been installed yet

$ IS_GRAPHQL_PPX_CI=true npm install
# Completes successfully

$ esy test
esy: error, exiting...
     invalid dependency ocaml: unable to resolve package
       processing package: graphql_ppx@0.0.4
       processing package: @opam/conf-m4@1

# Fails because npm install removed the OCaml stuff

$ esy install && esy build
# Completes successfully

$ esy test
...
/bin/sh: ../node_modules/.bin/bsb: No such file or directory
make: *** [test] Error 127

# Fails because esy install removed the npm stuff

[jordwalke]

I think that's because I didn't include bs-platform in the esy dependencies. I can try adding it there to see if it works, but the esy install and npm install aren't ever really supposed to happen with the same node modules. Esy just provides a better npm style workflow for building opam packages with opam dependencies. Then you can publish the result to npm without having esy be involved. I wanted esy support for this package because it makes my life way easier with the esy editor integration and install / build process during development. I would run the npm tests on pure npm for now.

[mhallin]

I incorporated @rgrinberg's comments on discover.ml and updated the DEVELOPING.md file with opam instructions.