Add sodium_crypto_secretbox_keygen function stub (#4954)

Author: thePanz

stubs/core.stub

@@ -52,6 +52,11 @@ function sodium_bin2base64(string $string, int $id): string {}
  */
 function sodium_bin2hex(string $string): string {}
 
+/**
+ * @return non-empty-string
+ */
+function sodium_crypto_secretbox_keygen(): string {}
+
 /**
  * @return ($string is non-empty-string ? non-empty-string : string)
  */