Add basic ls

src/user/app/echo/echo.c

@@ -1,7 +1,7 @@
 #include "src/lib/string.h"
 #include "src/user/syscall.h"
 
-int main(uint64_t argc, const char **argv) {
+uint64_t main(uint64_t argc, const char **argv) {
   for (uint64_t i = 1; i < argc; i++) {
     if (i > 1) {
       write(1, " ", 1);