Environment variables

To resolve an environment variable, js_of_ocaml will perform the following steps, in the given order:

- If the variable was set at compile time with the [–setenv VAR] flag, return it. - If running nodejs and the variable is set in the process environment ([process.env]), return it. - If the variable is set in [globalThis.jsoo_env], return it. This can be used to set an environment variable inside a web-browser. - Return Not_found