fix(exports): export usePreheader from package entry

Author: cossssmin

src/index.ts

@@ -34,6 +34,7 @@ export { useFont } from './composables/useFont.ts'
 export { useOutlookFallback } from './composables/useOutlookFallback.ts'
 export { useOutputPath } from './composables/useOutputPath.ts'
 export { usePlaintext } from './composables/usePlaintext.ts'
+export { usePreheader } from './composables/usePreheader.ts'
 export { useTransformers } from './composables/useTransformers.ts'
 export { useBaseUrl } from './composables/useBaseUrl.ts'
 export { useUrlQuery } from './composables/useUrlQuery.ts'