feat(realms): Add support for modules in realms.

[andreubotella]

TODO: Detailed description.

This PR is a port of denoland/deno#15760.

Towards #911.

[bartlomieju]

Alright, I think we can land this PR now :) awesome work @andreubotella.

@nayeemrmn please take a look as well

[nayeemrmn]

LGTM

[nayeemrmn]

Is there a problem with using preserve_snapshotted_modules from the runtime options on each realm? Since realms apply the snapshot implicitly and this is just another snapshot option.

[andreubotella]

I don't really have much of a clear idea at this point of what preserve_snapshotted_modules does in detail or how it's supposed to be used, other than that deno_runtime uses it for node: modules. I don't know if it makes sense to keep the option from runtime initialization.

Edit: The realm creation options let you pass a module loader, which will often be the same as the isolate's (that will be the case for ShadowRealm, for example), but for embedders it need not be (as <iframes> have different module maps in the web). I'm not too sure how that would interact with preserved_snapshotted_modules.

[nayeemrmn]

It makes all of the deno_<ext>: modules non-importable for users by removing them from the module map, but accepts a list of modules to exempt from that i.e. node: modules in our case. It's a step which is just as applicable to every realm.

[andreubotella]

While trying to understand how the snapshotted modules interact with the embedder-provided module loader, I noticed that currently they are not included at all in the realm's module map:

deno_core/core/runtime/jsruntime.rs

Line 871 in 444a0ae

// TODO(andreubotella): Should the module map be initialized with snapshotted data?

(that was added in PR #45, which was split off this one)

I will update this PR to include those snapshotted modules in the module map, and to support preserve_snapshotted_modules.

[andreubotella]

I think that should probably be a follow-up PR though.

[andreubotella]

I will revert the change that moves the code that handles preserved_snapshotted_modules inside init_extension_js, since it is now a lot clearer to me that it doesn't belong there. I will handle that in new_inner and create_realm in the follow-up PR.

[bartlomieju]

LGTM!