add importNowHook to SES Compartment

[naugtur]

What is the Problem Being Solved?

Any level of dynamic behavior for CJS require() in endo implemented on top of Compartment requires either preloading all dynamically requested modules or adding importNowHook - the synchronous version of importHook.

Description of the Design

Provide synchronous module loading facilities in SES by analogy to what already exists.

Security Considerations

TBD, none that I'm aware of.

Scaling Considerations

Test Plan

• port some unit tests to synchronous flow
• test with dynamic require in compartment-mapper

Compatibility Considerations

• This will never work with top-level await anywhere in the tree starting at the specifier being passed to importNow.

Upgrade Considerations

• We might have depended on a missing specifier causing a specific error from importNow (seems unlikely)