Dynamically link runtime js files

[gpetiot]

Hi, I'm taking over the work initiated by @jonludlam to make modules executables in toplevel (for the documentation on ocaml.org).

We were wondering if there was a way of dynamically linking the runtime.js files of some packages (eg. https://github.com/janestreet/base/blob/master/src/runtime.js) when initializing the toplevel, instead of when building the toplevel. It would be very nice to not have to build a new toplevel for each package.

I had a look at https://ocsigen.org/js_of_ocaml/latest/manual/runtime-files but it only mentions the build, and I didn't find anything in the jsoo cmdline or the API. Is that something possible? or is there an alternative? thanks.

(addressed by #1509)