Add support for specifying external helpers to avoid patching SWC

[xand3r40r93]

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.