breaking: require eslint v9 and switch to flat config

[jrmajor]

No description provided.

[benmccann]

@jrmajor I see that this PR is currently marked as a draft. GitHub doesn't notify when a PR is goes from draft to ready for review, so please ping me when you're ready for me to take a look and I will review it

[jrmajor]

@benmccann Ready for review. Please approve the workflow run in sveltejs/svelte#11108 to make sure that it works in the main Svelte repo, and read #32 before releasing a new version. FYI, typescript-eslint doesn't officially support ESLint v9 (typescript-eslint/typescript-eslint#8211), but it seems to work OK, so I don't think we need to wait for official support.

[benmccann]

Something else I was interested in trying was checking for promise-related errors. I just noticed in another project that TypeScript doesn't catch these types of errors. It should probably be another PR and isn't related to this change at all, but I thought I'd mention it just in case you were interested in working on it at all:

https://github.com/immich-app/immich/blob/7f504ec5fc6a300ec5265433a392f74db18c8e66/web/.eslintrc.cjs#L54