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_conf.h

@@ -72,7 +72,7 @@ typedef struct {
     nxt_mp_t             *pool;
     nxt_str_t            error;
     void                 *ctx;
-    nxt_array_t          *var_fields;  /* of nxt_var_field_t */
+    nxt_tstr_state_t     *tstr_state;
     nxt_mp_t             *conf_pool;
     nxt_uint_t           ver;
 } nxt_conf_validation_t;