Core: add generic handling of excluded locations

Currently there can be locations that are marked as excluded,
but don't have rules to enforce it, while fill has special handling
for excluded locations already.

This change removes special rules, and adds a generic rule instead.

docs/world api.md

@@ -102,7 +102,7 @@ Locations are places where items can be located in your game. This may be chests
 or boss drops for RPG-like games but could also be progress in a research tree.
 
 Each location has a `name` and an `id` (a.k.a. "code" or "address"), is placed
-in a Region and has access rules.
+in a Region, has access rules and a classification.
 The name needs to be unique in each game and must not be numeric (has to
 contain least 1 letter or symbol). The ID needs to be unique across all games
 and is best in the same range as the item IDs.
@@ -110,6 +110,10 @@ World-specific IDs are 1 to 2<sup>53</sup>-1, IDs ≤ 0 are global and reserved.
 
 Special locations with ID `None` can hold events.
 
+Classification is one of `LocationProgressType.DEFAULT`, `PRIORITY` or `EXCLUDED`.
+The Fill algorithm will fill priority first, giving higher chance of it being
+required, and not place progression or useful items in excluded locations.
+
 ### Items
 
 Items are all things that can "drop" for your game. This may be RPG items like