feat: add contains functions

string.c

@@ -139,4 +139,8 @@ char* str_trim(const char* str) {
     new_str[end-start+1] = '\0';
 
     return new_str;
-}
+}
+
+uint8_t str_contains(const char* str, const char* substr) {
+    return strstr(str, substr) != NULL;
+}