missing intellisence/autocomplete support for resource keys

[Xiaoy312]

What would you like to be added:

intellisence/autocomplete support for resource keys

Why is this needed:

general QoL

For which Platform: design-time editor

Anything else we need to know?

https://discord.com/channels/1182775715242967050/1182775715729526908/1308207632812740700

Does autocompletion work for anyone for material theme styles/colors/etc? i.e. stuff like this:

<... Background="{ThemeResource SurfaceBrush}" />

I haven't had that working in forever now, since like Uno 3
I need to look up the styles and names in uno docs whenever I want to reference something from the material theme
The material stuff just doesn't show in intellisense

the user claims that this used to work in the past, and broke for some reason
he hypothesized that could be when we promoted the main res-dict to a class

[Xiaoy312]

SO answer for wpf: https://stackoverflow.com/questions/49779105/xaml-intellisense-doesnt-work-for-referenced-assembly-resources
but we could inspire from this idea, and generate a "shopping list" res-dict that is only used during design-time