First-class support for init-only records in XAML

[maxkatz6]

Is your feature request related to a problem? Please describe.

Let's say we have a record defined as public record Model(int PropA, string PropB).
Currently developers have to use rather verbose syntax with x:Arguments, specifying each parameter ctor independently.

Describe the solution you'd like

Since this record is well defined in IL metadata via modreq(IsExternalInit) we can match these properties with type constructor, and include automatically transform into a proper constructor call using XML property attributes as parameters.

IntelliSense shouldn't be a problem, since these ctor parameters are already defined as mutable properties, making them visible for auto-completion engine.

Bindings won't be possible in this solution. Even if it's technically possible to setup binding for an init-only property, what would we do with initial ctor values?

Additional context

It's not a commonly used pattern with XAML apps. But can be well considered into complex custom controls. Something I thought from this example #18189.