Remove WORKER_SELF_REFERENCE in favor of new ctx.exports

[mhart]

A named binding is no longer needed: https://developers.cloudflare.com/changelog/2025-09-26-ctx-exports/

Currently requires the enable_ctx_exports compat flag

Happy to do a PR but my main questions would be around how to handle backwards compat, etc.

ie, straightforward to apple this to new projects, but how do we deal with existing projects that might be on an older compat date, etc?

[vicb]

Some thoughts:

cloudflare/workerd#5160 says "Due to some weird edge cases, we can just enable ctx.exports for everyone in prod". We should understand what those edge cases are before moving forward with this.

ctx.exports would not work in multiple workers setup, i.e. this. Arguably the name WORKER_SELF_REFERENCE is not appropriate in this case and we should change that.