Use dict_T to pass env vars to process spawning code

Co-authored-by: Matthieu Coudron <mattator@gmail.com>

src/nvim/eval/typval.c

@@ -1523,6 +1523,33 @@ varnumber_T tv_dict_get_number(const dict_T *const d, const char *const key)
   return tv_get_number(&di->di_tv);
 }
 
+/// Converts a dict to an environment
+///
+///
+char **tv_dict_to_env(dict_T *denv)
+{
+  size_t env_size = (size_t)tv_dict_len(denv);
+
+  size_t i = 0;
+  char **env = NULL;
+
+  // + 1 for NULL
+  env = xmalloc((env_size + 1) * sizeof(*env));
+
+  TV_DICT_ITER(denv, var, {
+    const char *str = tv_get_string(&var->di_tv);
+    assert(str);
+    size_t len = STRLEN(var->di_key) + strlen(str) + strlen("=") + 1;
+    env[i] = xmalloc(len);
+    snprintf(env[i], len, "%s=%s", (char *)var->di_key, str);
+    i++;
+  });
+
+  // must be null terminated
+  env[env_size] = NULL;
+  return env;
+}
+
 /// Get a string item from a dictionary
 ///
 /// @param[in]  d  Dictionary to get item from.