union #shared_nil

This adds a feature to `union` which requires all the variants to have a `nil` value and on assign to the union, checks whether that value is `nil` or not. If the value is `nil`, the union will be `nil` (thus sharing the `nil` value)

src/docs_format.cpp

@@ -99,6 +99,7 @@ enum OdinDocTypeFlag_Union : u32 {
 	OdinDocTypeFlag_Union_polymorphic = 1<<0,
 	OdinDocTypeFlag_Union_no_nil      = 1<<1,
 	OdinDocTypeFlag_Union_maybe       = 1<<2,
+	OdinDocTypeFlag_Union_shared_nil  = 1<<3,
 };
 
 enum OdinDocTypeFlag_Proc : u32 {