Adjust Pebble effects, add ranged MAGE attack, etc.

Adjusted various Pebble effects, added the "player_is_visible_from"
function and adjusted "determine_npc_behavior" to facilitate ranged
attacks from NPCs of type MAGE, modified "move_npc" to prevent NPCs from
standing on an EXIT, adjusted various constant names and values, and
updated the comments at the top of each source file.