fix(modernjs): add explicit return type to SSRLiveReload function

Fixes TypeScript error TS2742 where inferred type could not be named
without portable React types reference.

Author: ScriptedAlchemy

packages/modernjs/src/ssr-runtime/SSRLiveReload.tsx

@@ -1,4 +1,4 @@
-export function SSRLiveReload() {
+export function SSRLiveReload(): JSX.Element | null {
   if (process.env.NODE_ENV !== 'development') {
     return null;
   }