fix: preserve native Event/dispatchEvent in jsdom setup for Deno compatibility

[Lojhan]

Problem

When running tests under Deno with happy-dom, the process exited with:

TypeError: Failed to execute 'dispatchEvent' on 'EventTarget': parameter 1 is not of type 'Event'.
    at GlobalWindow.dispatchEvent (happy-dom/src/event/EventTarget.ts:122:10)
    at dispatchBeforeUnloadEvent (ext:runtime_main/js/99_main.js:469:22)

With jsdom + isolation:process, each per-test subprocess exited with:

TypeError: Cannot set properties of undefined (setting 'target')
    at setTarget (ext:deno_web/02_event.js:97:29)
    at dispatchEvent (ext:deno_web/02_event.js:1055:7)
    at dispatchLoadEvent (ext:runtime_main/js/99_main.js:466:15)

Both errors share the same root cause: dom-env.ts was overwriting globalThis.Event, globalThis.CustomEvent, and globalThis.dispatchEvent with the DOM library's own implementations. When Deno's runtime teardown fires dispatchLoadEvent() on exit it uses its own native Event constructor, which is no longer present on globalThis.

isolation:none was unaffected because no subprocess teardown occurs.

Fix

In both setupHappyDomEnvironment() and setupJsdomEnvironment() in dom-env.ts:

1. Capture the runtime's native Event, CustomEvent, and dispatchEvent before installing the DOM library globals.
2. After installation, restore them so Deno's exit handlers see the original constructors.

This leaves full DOM functionality intact while keeping Deno's shutdown path working.

Compatibility matrix after fix

All 24 combinations now pass (Node × Bun × Deno) × (happy-dom × jsdom) × (isolation:none × isolation:process) — 24/24 ✅.