Don't show hover cards for primitive types

[elliette]

Hovering over field with a primitive type doesn't provide any extra information. Eg:

It might make sense to never show a hovercard for a primitive type value. We could also change the cursor style to indicate whether hovering will provide any extra info or not.

[elliette]

This would also improve some of the performance issues we've seen inspecting Flutter Web apps, since less expression evaluations would need to be done. FYI @CoderDake @annagrin