Provide an option to dump environment to JSON

The way dependencyEnv currently works is hard to integrate into editor plugins.

See my earlier comment here.

It would be ideal if dependencyEnv could just produce a JSON object that can be used directly with child_process.spawn. The current output is not usable for this without parsing it and then normalizing it (e.g., interpolation).

The way I am currently integrating dependencyEnv into vscode is by spawning a composite command with sh -c. However, this causes a number of reliability problems.

[jordwalke]

Cc @andreypopp.
I'll add the issue to esy here: jordwalke/esy-issues#22