feat(Chat): Add .message_stream_context() (#1906)

Author: cpsievert

CHANGELOG.md

@@ -11,6 +11,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Both `ui.Chat()` and `ui.MarkdownStream()` now support arbirary Shiny UI elements inside of messages. This allows for gathering input from the user (e.g., `ui.input_select()`), displaying of rich output (e.g., `render.DataGrid()`), and more. (#1868)
 
+* Added a new `.message_stream_context()` method to `ui.Chat()`. This context manager is a useful alternative to `.append_message_stream()` when you want to: (1) Nest a stream within another and/or
+(2) Overwrite/replace streaming content. (#1906)
+
 ### Changes
 
 * Express mode's `app_opts()` requires all arguments to be keyword-only. If you are using positional arguments, you will need to update your code. (#1895)