u16 scalar usage in the indexer

[Braqzen]

In the scalar section of the book there are multiple types but the u16 is not mentioned.

Sway supports the type and so does the indexer because we can build an indexer without any compilation errors.
The issue is that the type is undocumented so I'm unsure how to correctly capture that data.

I assume we could use a larger type, U32, but that's counter-intuitive.