Infer output formats from package.json

When no `--format` flag is specified..
```sh
tsup src/index.ts
```
..tsup can look in `package.json` for clues.

- The `main`, `module`, `types`/`typings`, and `unpkg`/`jsdelivr` fields all imply the preferred output paths, even allowing for custom bundle names.
- When `module` exists, tsup creates an ESM bundle.
- When `types` or `typings` exist, tsup creates a `.d.ts` bundle.
- When `unpkg` or `jsdelivr` exist, tsup creates a UMD bundle.