Extract runtime functionality into separate Runtime_* modules (part 1)

[cknitt]

Currently, the compiler relies on the presence of the Js and Belt modules for some runtime functionality. However, we are moving to Core as our new standard library and should therefore not rely on Js or Belt being present (these will likely become optional packages at some point). See also #6836.

Therefore, any runtime functionality the compiler relies on should be extracted into separate modules for this purpose. I have used the naming convention runtime_*.res for these modules. They currently remain in the others directory because for the source files in the existing runtime directory (caml_*.res), the corresponding .cm* files are not part of the npm package.

This PR is a first part of the required work.

[cristianoc]

Looks great.
Something to keep in mind is some of these paths are used in the vscode extension.
See e.g. Packages.ml:

                      promiseModulePath = ["Js"; "Promise"];

So I'd say do a quick grep in that repo, and mark all the relevant paths that have changes in an issue on the rescript-vscode repo so they can be adapted at some point.

CC @zth

[cristianoc]

Btw, in this case Promise.import seems something where no action is required on the extension side, just it won't appear in autocompletion anymore, which is a good thing. But, it's good to create an issue so it can be checked properly.

[cknitt]

So I'd say do a quick grep in that repo, and mark all the relevant paths that have changes in an issue on the rescript-vscode repo so they can be adapted at some point.

The Js modules are not going away yet though. So I think for now nothing should break in rescript-vscode?
I think it will be enough to review rescript-vscode at the point when the Js and Belt modules become optional?