fix: export webpack helper subpath

[xianzuyang9-blip]

Fixes #60.

What changed

• Exported the packaged webpack helper at vortex-api/bin/webpack.
• Also exported the explicit vortex-api/bin/webpack.js subpath for consumers that include the extension.

The helper is already included in the npm package via files: ["./bin", ...] and the migration guide still documents the webpack helper path, but the v2 export map only exposed the root typing entrypoint. That makes Node reject existing webpack configs with ERR_PACKAGE_PATH_NOT_EXPORTED.

Validation

Before this change, this fails with ERR_PACKAGE_PATH_NOT_EXPORTED:

node -e "require.resolve('vortex-api/bin/webpack', { paths: [process.cwd()] })"

After this change, these pass:

node -e "console.log(require.resolve('vortex-api/bin/webpack', { paths: [process.cwd()] }))"
node -e "console.log(require.resolve('vortex-api/bin/webpack.js', { paths: [process.cwd()] }))"
node -e "const webpack=require('vortex-api/bin/webpack').default; console.log(typeof webpack);"
npm pack --dry-run