[codex] split Event owner types without ReScript 13

[jderochervlk]

Summary

This separates the Event API split from the ReScript 13 upgrade work in PR #282.

• Adds public owner modules/types for Event.t, EventTarget.t, EventType.t, AbortSignal.t, and ExtendableEvent.t.
• Updates dependent Web API records and event listener call sites to use those owner types.
• Keeps the package on ReScript >=12.0.0 <13 and leaves iterator-return APIs on the current ReScript 12 shape.
• Keeps Window.current usage instead of the Base.window shape from the ReScript 13-oriented PR branch.

Validation

• npm test
• npm run format:check

Notes

package-lock.json had a pre-existing local modification in the worktree and was intentionally left out of this PR.