Support for nullable/optional value types

[lemonmojo]

ie. bool TryGetId(out Guid? id)

I think that's currently an unsupported scenario.
Have to check.

[lemonmojo]

So the reason is that under the cover it's actually a generic Nullable<T>: https://learn.microsoft.com/en-us/dotnet/api/system.nullable-1?view=net-7.0

[lemonmojo]

Example of what such a parameter looks like:

[lemonmojo]

Same applies to primitives btw. ie. int?/Nullable<int>.

[lemonmojo]

Partial Strategy: Add an extension method to System.Type: bool IsNullableValueType(this Type type, out Type? valueType) so that we can figure out if the targeted type is a nullable value type and if it is, get the underlying type as an out parameter.

As a first step, this information can be used to emit a more detailed unsupported type reason message.

See https://learn.microsoft.com/en-us/dotnet/api/system.nullable.getunderlyingtype?view=net-7.0

[lemonmojo]

I think that once a TypeDescriptor is created for such a type, we can add conversions there which will then be used by later generation phases.

[lemonmojo]

fc54eec added support for nullable structs (no support yet for nullable primitives and enums).