Jsonrpc: rework fix for bug#60088

Jsonrpc: rework fix for bug#60088

Try to decouple receiving text and processing messages in the event
loop.  This should allow for requests within requests in both Eglot
and the Dape extension (https://github.com/svaante/dape).

jsonrpc-connection-receive is now called from timers after the process
filter finished.  Because of this, a detail is that any serialization
errors are now thrown from timers instead of the synchronous process
filter, and there's no good way to test this in ert, so a test has
been deleted.

* lisp/jsonrpc.el (jsonrpc--process-filter): Rework.

* test/lisp/jsonrpc-tests.el (json-el-cant-serialize-this): Delete test.