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_main.h

@@ -68,6 +68,7 @@ typedef uint16_t                     nxt_port_id_t;
 #include <nxt_sprintf.h>
 #include <nxt_parse.h>
 #include <nxt_var.h>
+#include <nxt_tstr.h>
 
 
 /* TODO: remove unused */