NODE_CODE_CACHING with dynamic libraries

Hi there,

Here is a feature request:

I am working on a NodeJS application, which starts up a relatively large Emscripten / WASM project, which uses several dynamically linked libraries (via the `dynamicLibraries` entry in the `Module` object). Currently, startup of the application takes several minutes. To speed things up, I imagine it would be very helpful to use `NODE_CODE_CACHING=1`. However, this only seems to work with the `MAIN_MODULE` (I get a `*.cache.xxxxx` file only for that type of module) and not with any `SIDE_MODULE` (there are no `*.cache.xxxxx` files).