Allow webs to be placed without a giant spider on them

Allow creating webs without spiders in the lua level scripts:

des.trap({ type = "web", spider_on_web = 0 });

Based on xNetHack commit by copperwater <aosdict@gmail.com>.

Also changes the Spider nest themed room to generate without
spiders when the level difficulty is 8 or less.

include/sp_lev.h

@@ -127,6 +127,7 @@ typedef struct {
 typedef struct {
     packed_coord coord;
     xchar x, y, type;
+    boolean spider_on_web;
 } spltrap;
 
 typedef struct {