Abstract DAP handler (#1170)

Progress towards posit-dev/positron#12104

We're going to have two handlers for DAP messages, one through the TCP
connection for Console debugging, and one through a Jupyter connection
(Control socket) for Notebook debugging.

This PR abstracts out DAP handling to make it agnostic to the transport:
a `DapHandler` takes a DAP message, handles it, and returns a
`Result<DapHandlerOutput>`. `dispatch()` is in charge of transforming
error cases to a DAP response (success or error). The caller (transport
layer) then propagates via appropriate transport.

Evaluate remains tied to the TCP transport because it needs to run R
code asynchronously, and Jupyter's Control socket requires quick
synchronous replies.

This PR is pure reorganisation, no functional difference.

Author: lionel-