Deprecate `@ampproject/remapping`

The repo for https://www.npmjs.com/package/@ampproject/remapping has been deprecated, but essentially no one has migrated to `@jridgewell/remapping` as it's not obvious a new package has been available. We just switched Svelte and vitest, so some downloads should start hitting the new package soon, but I think it's quite hard for other projects to discover that they should move