convert @babel/helper-split-export-declaration to TypeScript (#12485)

Co-authored-by: Nicolò Ribaudo <nicolo.ribaudo@gmail.com>

lib/babel-packages.js.flow

@@ -164,3 +164,7 @@ declare module "@babel/helper-function-name" {
     localBinding?: boolean
   ): any;
 }
+
+declare module "@babel/helper-split-export-declaration" {
+  declare export default function splitExportDeclaration(exportDeclaration: any): any;
+}