feat(eventsource): remove event source package from monorepo

[RitaDias]

Description

Move eventsource from monorepo to avoid circular dependencies between the Client and the Sanity Studio code. It has now been moved to sanity-io/eventsource

Currently:
Client depends on EventSource (monorepo)
Monorepo (multiple packages) depends on Client & EventSource

So when you update the EventSource, then you need to update the Client and in turn you need to update the Monorepo again

What to review

Removal of the code and checking in with the other git repo

Notes for release

Moved EventSource package outside of the monorepo.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployments, click below or on the icon next to each commit.

perf-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/perf-studio/CXbnLvtfqBqxqmB9GpWJuV5RJ4HB
✅ Preview: https://perf-studio-git-remove-event-source-from-monorepo.sanity.build

test-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/test-studio/EQJBGuDY5328g34o9YCb94bmvbbY
✅ Preview: https://test-studio-git-remove-event-source-from-monorepo.sanity.build

studio-workshop – ./dev/workshop

🔍 Inspect: https://vercel.com/sanity-io/studio-workshop/61pnr5WwQKaQXLzxB6cNQ1FroK54
✅ Preview: https://studio-workshop-git-remove-event-source-from-monorepo.sanity.build

[rexxars]

• The removal looks good (well, duh)
• The new repo should have an updated repository field in package.json (remove directory field, update url)

When we switch to the new polyfill, make sure to bump the version to 3.0.0 to indicate the change in behavior (eg always using polyfill)