Promise onDispose callback

Hi! I had a small naive question,

Since `ctx.dispose()` is a Promise, would it be possible to allow ESbuild to wait for plugins as well when disposing? The possibility to add promises to the [dispose callback array](https://github.com/evanw/esbuild/blob/5e0e56d6d62076dfeff47f5227ae5300f91d2b16/lib/shared/common.ts#L1328C1-L1330C11) would allow developers to await for all plugins to be cleaned up until proceeding.

Start in lib/shared/common.ts at the dispose callback array linked in the issue, then trace how ctx.dispose() processes those callbacks. Determine the API and lifecycle changes needed for promise-returning plugin cleanup, and verify that ctx.dispose() waits for all registered cleanup before resolving.

Written by the indexing model from the issue text.