Improve discoverability of API methods

the `api` object is filled with auto-generated methods.
It makes some sense once you've learnt the details but is not that great when you haven't.

`api.query` - reads from storage
`api.tx` - makes a transaction
`api.derive` - generally compound queries, could do anything read/write
`api.rpc` - generally triggers calculations + reads on node side, can also do things like test run a contract, set local node session keys

We could look to group & present these in a nicer way to the user.
Might allow us some affordance to breaking changes also if the underlying generated api changes

cc @Amy-Centrality