Forbid top-level-await in __syncModuleProgram__ module source protocol

[kriskowal]

Describe the bug

Currently, bundle-source (via @endo/module-source) will happily generate a bundle where a module contains a top-level-await which defers a SyntaxError to runtime, since the enclosing function is not async.

Steps to reproduce

bundle-source entry.mjs where entry.mjs is await null, then run this bundle with importBundle.

Expected behavior

bundle-source should balk because ModuleSource throws a user-meaningful exception like “Endo emulated ModuleSource does not yet support top-level-await”.

Platform environment

Endo at 271aa92