A hook that runs once after all builds have completed

## Current behavior

The `onSuccess` hook runs once for every build config. In this example, `./prepareNpmEntrypoints.sh` will run twice.

```ts
// tsup.config.ts
export default defineConfig([
{ /* ... */ },
{ /* ... */ },
]);
```

```bash
pnpm tsup --watch --onSuccess "./prepareNpmEntrypoints.sh"
```

## Feature request

A hook that runs once, when all of the builds have completed. This can be used, for instance, to run a script that patches up the build products in some way that's not possible to do _during_ the build.

Start with the multi-config example in tsup.config.ts and run pnpm tsup --watch using the onSuccess command shown. Trace how the current hook is invoked for each build configuration, then determine where an all-builds-completed hook belongs; done means ./prepareNpmEntrypoints.sh runs once after the complete build set rather than once per configuration.

Written by the indexing model from the issue text.