Add support for specifying external helpers to avoid patching SWC

Currently, in order to enable externalHelpers for legacy decorators in SWC, users need to manually patch swc.transformFile. It would be helpful if tsup provided a way to specify this option without requiring modifications to SWC itself.
### Example workaround:
```
const transformFile = swc.transformFile;
swc.transformFile = (filePath, options) => {
if (options?.jsc?.transform?.legacyDecorator) options.jsc.externalHelpers = true;
return transformFile.call(this, filePath, options);
};
```
### Proposed solution:
Introduce an option in tsup to allow enabling externalHelpers when using decorators, eliminating the need for manual patches.

Start by tracing tsup's SWC integration around swc.transformFile and the decorator-related options shown in the workaround. Define where the new configuration belongs, how it should enable externalHelpers only for the relevant decorator flow, and how the behavior can be verified; the issue names no repository files or tests, so those entry points must first be located.

Written by the indexing model from the issue text.