Avoid `instanceof` and remove the "node" export condition

[timostamm]

In connectrpc/connect-es#974, we override ConnectError's implementation of instanceof to avoid the dual package hazard, which allows us to remove the "node" export condition, which causes problems with some bundlers.

We need to take a similar approach here: Avoid the dual package hazard with instanceof, and remove the "node" export condition.

There are many more uses of instanceof here that in Connect-ES, and we will need to take a more cautious approach, and also make sure that we do not regress on performance.