[C#] Support System.Collections.Immutable collections as custom containers

[enioluwas]

C# immutable collections are useful data types for building truly immutable Bond structs.

Bond doesn't properly support using them as custom type aliases, for the following reasons:

1. Generated classes always use the new ImmutableX() as the default constructor, but those do not exist for immutable collections; they are instead constructed with the static field ImmutableX<T>.Empty.
2. Serialization works, but deserialization fails since the compiler assumes that all BT_LIST/BT_SET aliases properly implement ICollection<T>.Add(), but all mutation methods from ICollection<T> throw a NotSupportedException for immutable collections. Same issue with BT_MAP, but with the setter IDictionary<K, V>.Item[].

The desired mappings would be:

BT_LIST -> ImmutableArray<T>, ImmutableList<T>

BT_SET -> ImmutableHashSet<T>, ImmutableSortedSet<T>

BT_MAP -> ImmutableDictionary<T>, ImmutableSortedDictionary<T>

Based on my initial investigation, the following would need to be done:

1. Modify the C# compiler to detect immutable collections and use the static field to construct them, rather than a parameterless constructor.
2. Modify the C# deserializer to specially handle reconstructing immutable collections, either using their builders as an intermediary, using the specific Add methods for the BT_LIST/BT_SET objects, and the SetItem method for the BT_MAP objects.