Var: separating nxt_tstr_t from nxt_var_t.

It's for the introduction of njs support.
For each option that supports native variable and JS template literals introduced next,
it's unified as template string.

No functional changes.

src/nxt_router.c

@@ -1060,8 +1060,8 @@ nxt_router_temp_conf(nxt_task_t *task)
 
     rtcf->mem_pool = mp;
 
-    rtcf->var_fields = nxt_array_create(mp, 4, sizeof(nxt_var_field_t));
-    if (nxt_slow_path(rtcf->var_fields == NULL)) {
+    rtcf->tstr_state = nxt_tstr_state_new(mp);
+    if (nxt_slow_path(rtcf->tstr_state == NULL)) {
         goto fail;
     }