feat(server-utils): initial module scaffolding

[isaacs]

This is phase 1 of the plan to move all server utils into a single reusable package that can be shared across SSJS SDKs.

Right now it does not do anything, but in subsequent steps:

1. Code will be moved out of @sentry/core/server, into @sentry-internal/server-utils.
2. @sentry/core/server will live on, but re-export the contents of the server-utils module.
3. To minimize the cyclical dependency (ie, server-utils depends on core for isomorphic utilities, core depends on server-utils for ./server exports), server-utils will have a types-only dependency on core, which can be removed in v11. For now, an nx override allows the cycle explicitly.

[isaacs]

Actually it looks like this might not be possible without a breaking change, because the cycles will end up confounding TSC when it builds our code, as core and server-utils would depend on one another for types. The only option would be to shim everything, which gets pretty gross.