Disable top-level await in require with a compat flag

[jasnell]

Currently, the top level scope of a Worker script and modules that are imported may use top level await. However, because we restrict the use of i/o in the top level scope, the only thing that can be awaited at the top level scope is a dynamic import, which we require to be resolved synchronously. It is a bit nonsensical to support synchronously resolved dynamic imports at the top level when we can just use static imports more effectively. Additionally, the ecosystem and runtimes are moving to a state where top level await in modules is being strongly discouraged. This compat flag. This compatibility flag will make it impossible to use top level await.

(made the change with lots of distractions so keeping as a draft until I can do a second pass to make sure nothing was missed)

Update: Updated to apply the compat flag only to require() calls, along with a few other relevant cleanups. Still keeping as a draft because this will need some extensive testing.

[jasnell]

After an initial review of impact on this, I think this PR needs to be updated to relax the restriction a bit more. TLA should be restricted only when modules are require(...), but entry point and import would work fine.

[jasnell]

Updated to further refine and also eliminates the limitation around circular dependencies of commonjs modules. Not marking this "ready for review" just yet because there are a few more additional cleanups I want to do to eliminate duplicated code.

[jasnell]

@kentonv

There are actually several async APIs that work at startup time, including WebCrypto and WebAssembly. I think there are real use cases for being able to use these at startup so I'm not excited about taking it away.

Those will not be impacted. Note that this PR has been updated to ONLY apply to use of require() and createRequire(). There is zero impact on the worker top-level evaluation and anything imported using static or dynamic import.