[FEATURE] improve getCloudflareContext()

[vicb]

getClouflareContext retrieves the context from either a worker request (through ALS) or by starting a getPlatformProxy in next dev.

There are a few things we could improve:

• throw in a worker context when getClouflareContext is called outside of a request context (i.e. top level)
• remove the node specific code in the worker context (we could define WORKER_CONTEXT = true in esbuild to strip the code at build time).

See:

• https://github.com/cloudflare/next-on-pages/blob/2cd4c3c704a00e6b693229f1f14102abc6318d11/packages/next-on-pages/src/api/getRequestContext.ts#L58
• https://github.com/cloudflare/next-on-pages/blob/2cd4c3c704a00e6b693229f1f14102abc6318d11/packages/next-on-pages/src/api/getRequestContext.ts#L62

/cc @dario-piotrowicz @james-elicx