Add singleton API to World

- singleton() combines component() with set()
- get() looks up a singleton component value
- set() sets it, though singleton must exist before

src/world.zig

@@ -95,6 +95,41 @@ pub fn World(comptime ctx: anytype) type {
             return Entity(ctx).fromRaw(self, result);
         }
 
+        /// Registers a singleton component of type `T` with the specified value.
+        ///
+        /// A singleton is a component which has itself added to its entity.
+        /// This allows looking up a value of `T` using its own type. Only one
+        /// singleton of each type can exist per world, but other entities may
+        /// still have this component added to them.
+        ///
+        /// Use `get()` and `set()` to get and set the value of this singleton.
+        ///
+        /// Returns the created component entity.
+        pub fn singleton(self: *Self, comptime T: type, value: T) !Entity(ctx) {
+            const single = try component(self, T);
+            single.set(T, value);
+            return single;
+        }
+
+        /// Gets the value of the singleton component with type `T`.
+        ///
+        /// Returns `error.IsNotAlive` if the entity is missing.
+        /// Returns `error.ComponentMissing` if the component is missing.
+        /// Both of these may occur from not calling `singleton()` first.
+        pub fn get(self: *Self, comptime T: type) !T {
+            const e = try self.lookupByType(T);
+            return e.get(T) orelse error.ComponentMissing;
+        }
+
+        /// Sets the value of the singleton component with type `T`.
+        ///
+        /// Returns `error.IsNotAlive` if the entity is missing.
+        /// This may occur from not calling `singleton()` first.
+        pub fn set(self: *Self, comptime T: type, value: T) !void {
+            const e = try self.lookupByType(T);
+            e.set(T, value);
+        }
+
         pub fn system(
             self: *Self,
             name: [:0]const u8,