feat(r): standalone register_slash_command()

[kaipingyang]

Fixes the standing TODO in chat_app.R for a standalone register_slash_command(). See #273 for motivation and the proposed API.

What this does

Factors the slash-command registration machinery out of chat_server() into:

• register_slash_command(id, name, description, handler, ..., echo = NULL, force = FALSE, session = getDefaultReactiveDomain()) — exported. A free function you can call from anywhere with just the chat id and session; no need to thread chat_server()'s return value, and it works with a fully custom chat_ui() + chat_append() loop (no chat_server() required).
• slash_commands_registry(id, session) — internal. Get-or-create the per-(session, id) command registry, wiring the dispatch + sync observers exactly once. State lives in session$userData keyed by the namespaced id, matching the chat_restore.R idiom.

chat_server()'s $slash_command() is now a thin wrapper over register_slash_command(), so both paths share one implementation. This is a pure refactor — no behavior change to the existing chat_server() API.

Semantics (unchanged from $slash_command())

• handler: 0-/1-arg function, or NULL for client-side handling via the cancelable shiny:chat-slash-command event. A 1-arg handler receives a ContentSlashCommand.
• echo defaults to TRUE when a handler is supplied, FALSE otherwise.
• force overwrites an existing command; returns an unregister function.
• Argument order follows chat_append() / chat_clear() (id first, session last).

Test plan

• Existing chat_server slash tests still pass (updated the 3 that reached into the old closure to read the registry from its new location).
• New register_slash_command() tests: 0-/1-arg dispatch, ContentSlashCommand payload, validation, echo defaults, force, unregister, single-registry-per-(session, id).
• Full suite green (devtools::test(), 516 tests, 0 failures).
• R CMD check: 0 errors, 0 warnings (remaining NOTES are pre-existing and unrelated).
• Verified end-to-end in a real headless browser: the palette renders and commands dispatch for both chat_server() and standalone register_slash_command(), including inside a Shiny module, the 0-arg path, the client-side NULL-handler + preventDefault() path, and the content@text → chat_append() pattern.

Open question

I kept the unregister-closure return (R idiom). If you'd rather have a remove_slash_command() for parity with Python's Chat.remove_slash_command(), I'm happy to add it.