Tools: enable doc extraction from compiler

[aspeddro]

This PR has one workaround to extract docs from the compiler repo and a fix

1. Introduce an env var FROM_COMPILER to analysis find the correct path to cmt files. In standalone packages, the cmt files are located in lib/bs/, but in the compiler repo they are in lib/ocaml
2. Add stdio: "inherit". Node was truncating output when I run a separate process in rescript-tools doc path/to/rescript-compiler/lib/ocaml/js.ml child process output truncated nodejs/node#2360

[zth]

What about doing it as a regular variable to the cli command instead of an env variable?

[aspeddro]

This is just to generate the compiler docs. It is an internal and exclusive use for extracting compiler docstrings in the rescript-lang repo via the rescript-tools package

I don't think it would make sense to other users

[zth]

@aspeddro ok, sounds good! Is this ready to go then?

[aspeddro]

yup. CHANGELOG updated