feat(core): Wrap ExchangeIO in share calls in composeExchanges (QOL)

[kitten]

Summary

This is a small quality of life improvement for the composeExchanges helper in @urql/core that should make it easier for users to write and understand Exchanges.
It simply removes the need for share to be called manually on Exchanges by wrapping the ExchangeIO function in share calls automatically.

This shortens our docs on "Authoring Exchanges" by a whole large section, simplifies some exchanges, and shouldn't have any negative consequences, except for mocked ExchangeIO in tests requiring a share call now.

Set of changes

• Wrap forward's argument and return value in share in composeExchanges
• Add invariant in development that enforces exchanges to call forward just once
• Update all exchanges to remove redundant share calls