in script/build.ts [138:148]
async function writeMainTsIndex(types: string[]) {
let output = ''
for (const type of types) {
output += `import ${type} from './${type}'\n`
}
output += `export default { ${types.join(', ')} }`
const dir = path.join(tsDir)
await mkdirp(dir)
fs.writeFileSync(path.join(dir, `index.ts`), output)
}