Introduce --nodejs_compat_unleashed runtime flag

[IgorMinar]

As we improve our node.js compat story, it would be great have a runtime flag that offers a more "aggressive" and compatible version of --nodejs_compat runtime flag. We could call this flag --node_compat_unleashed or similar.

This new flag should expose supported node builtin modules without the node: prefix, and should also expose globals like process, Buffer on globalThis.

We can then test with this flag, and at some point in the near future, this flag would be activated via a compat date as long as nodejs_compat is present (essentially making the existing nodejs_compat become nodejs_compat_unleashed once the compatibility date is reached or exceeded.

Originally proposed in https://chat.google.com/room/AAAAZwUYa48/4rD9Yjauj9w/yE5EU56IaRg?cls=10

[kentonv]

Bikeshed comment: The name "unleashed" sounds a little, uh, silly to me. Maybe nodejs_compat_no_prefix or _no_namespace or something?

[IgorMinar]

I'm not attached to the name. The flag should in addition to unprefixing modules also introduce node's globals, so no_prefix seems not comprehensive enough.

[jasnell]

heh, yeah, I wasn't going to go with "unleashed" ;-) ... I'll be working on this in the coming couple of weeks, want to get the Request.cache stuff done first, then will do this next.

[jasnell]

Initial draft implementation in #2147