[fix] add CustomEvent as a known global

[bluwy]

Trying to dispatch a new CustomEvent without warning. Not sure if we need to add the other types of events to the list, it's probably rare to do a new MouseEvent.

Before submitting the PR, please make sure you do the following

• [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• [x] Prefix your PR title with [feat], [fix], [chore], or [docs].
• [ ] This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests

• [ ] Run the tests with npm test and lint the project with npm run lint

[baseballyama]

Close in favor of https://github.com/sveltejs/svelte/pull/7786

Thank you for your review @bluwy !