Add --rpc option for roc repl

[ageron]

This PR adds the --rpc option to roc repl.
It starts the REPL in RPC mode, where the inputs and outputs are JSON objects. For example {"method": "repl.start"}.

Methods:

• repl.start
• repl.stop
• repl.reset
• repl.evaluate
• repl.autocomplete
• repl.inspect

For example:

% roc repl --rpc
{"method": "repl.start", "id": "req001"}
{"session_id":"68da396c-9302-4d2a-a800-a3b644665c68","id":"req001"}
{"method": "repl.evaluate", "params": {"session_id": "68da396c-9302-4d2a-a800-a3b644665c68", "code": "2 + 2"}, "id": "req003"}
{"result":"4.0","stdout":"","diagnostics":null,"id":"req003"}
{"method": "repl.stop", "params": {"session_id": "68da396c-9302-4d2a-a800-a3b644665c68"}, "id": "req004"}
{"success":true,"id":"req004"}

[rtfeldman]

@ageron hm, I'd really rather not introduce JSON into any of our CLI or compiler outputs... 😅

Is there another way we could do this?

[ageron]

Hi @rtfeldman ,
Isn't that how the LSP works? If we want external tools to provide a REPL UI (such as Jupyter Notebook), then we need some kind of programmatic interface. We could just parse the REPL UI, but that's pretty brittle (e.g., if a program outputs » the tool will think it's a prompt). Let's continue this discussion on Zulip.

[hawkinsw]

4122 is technically obsoleted by https://datatracker.ietf.org/doc/rfc9562/.