feat: added contains check to observable lists

src/observablecollection/src/Shared/ObservableList.lua

@@ -323,6 +323,26 @@ function ObservableList.Get<T>(self: ObservableList<T>, index: number): T?
 	return self._contents[key]
 end
 
+--[=[
+	Checks if the list contains a value
+
+	:::warning
+	This can scan the whole list each call; cost is linear in the number of entries.
+	:::
+
+	@param value T
+	@return boolean
+]=]
+function ObservableList.Contains<T>(self: ObservableList<T>, value: T): boolean
+	for _, item in self._contents do
+		if item == value then
+			return true
+		end
+	end
+
+	return false
+end
+
 --[=[
 	Adds the item to the list at the specified index
 	@param item T

src/observablecollection/src/Shared/ObservableList.spec.lua

@@ -35,6 +35,10 @@ describe("ObservableList.new()", function()
 		expect(observableList:Get(-2)).toEqual(nil)
 	end)
 
+	it("should check if the list contains a value", function()
+		expect(observableList:Contains("a")).toEqual(true)
+	end)
+
 	it("should allow false as a value", function()
 		expect(observableList:Get(2)).toEqual(nil)
 		observableList:Add(false)