Support JsonElement <--> IAttributeTable (#113)

This was done by renaming what used to be called `StjAttributesTable` and exposing it publicly.

Also:

- `System.Text.Json` reference version is high enough that we can use the more efficient deserialization from `JsonElement`.
- Recreate the extension methods as instance methods on the class, and mark the extension methods themselves as obsolete.
- Add some parameter validation for situations that we previously assumed could never happen.

Resolves #60