fix(wasm) add common definitions to stdlib (#5199)

Also expose `strlen` through `string.h` instead of `stdio.h`.

crates/language/wasm/src/stdio.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <string.h>
 
 typedef struct {
   bool left_justify;   // -
@@ -105,12 +106,6 @@ static int ptr_to_str(void *ptr, char *buffer) {
   return 2 + len;
 }
 
-size_t strlen(const char *str) {
-  const char *s = str;
-  while (*s) s++;
-  return s - str;
-}
-
 char *strncpy(char *dest, const char *src, size_t n) {
   char *d = dest;
   const char *s = src;