fix(src/core/index): Made it a tsx so that it can be compiled even if the component is exported.

Author: refirst11

src/core/index.ts src/core/index.tsxsrc/core/index.ts renamed to src/core/index.tsx

@@ -1,4 +1,4 @@
 export * from './media';
-export { PreviewServerCSS } from '../_internal';
+export { PreviewServerCSS } from './component/server-css';
 import Style from './style';
 export default Style;