feat(chat_mod_server): Add $append() function

[gadenbuie]

Adds $append() to the functions returned by chat_mod_server() to make it easier to extend a chat UI module. The implementation simply wraps chat_append() with id and session provided.

Closes #140

Example

library(shiny)
library(bslib)
# pak::pak("posit-dev/shinychat/pkg-r#145")
pkgload::load_all(here::here("pkg-r"))
library(shinychat)

ui <- page_fillable(
  chat_mod_ui(
    "chat",
    messages = list(
      list(role = "assistant", content = "👋 Hello and welcome to the chat!")
    )
  ),
  layout_columns(
    actionButton("reset_chat", "Reset chat"),
    actionButton("append_chat", "Append to chat")
  )
)

server <- function(input, output, session) {
  client <- ellmer::chat("openai/gpt-4.1-nano")
  
  chat <- chat_mod_server("chat", client)
  
  observeEvent(input$reset_chat, {
    chat$clear(
      "🔄 Chat was reset. Ready for new dialog!",
      client_history = "clear"
    )
  })
  
  observeEvent(input$append_chat, {
    chat$append("🫡 This is an appended message. Woot!")
  })
}

shinyApp(ui, server)