Add agent: chat with follow action (experimental) (#31401)

This PR introduces a new `agent: chat with follow` action that
automatically enables "Follow Agent" when submitting a chat message with
`cmd-enter` or `ctrl-enter`. This is experimental. I'm not super
thrilled with the name, but the root action to submit a chat is called
`agent: chat`, so I'm following that wording. I'm also unsure if the
binding feels right or not.

Release Notes:

- Added an `agent: chat with follow` action via `cmd-enter` on macOS and
`ctrl-enter` on Linux

assets/keymaps/default-linux.json

@@ -274,6 +274,7 @@
     "context": "MessageEditor > Editor",
     "bindings": {
       "enter": "agent::Chat",
+      "ctrl-enter": "agent::ChatWithFollow",
       "ctrl-i": "agent::ToggleProfileSelector",
       "shift-ctrl-r": "agent::OpenAgentDiff"
     }

assets/keymaps/default-macos.json

@@ -311,6 +311,7 @@
     "use_key_equivalents": true,
     "bindings": {
       "enter": "agent::Chat",
+      "cmd-enter": "agent::ChatWithFollow",
       "cmd-i": "agent::ToggleProfileSelector",
       "shift-ctrl-r": "agent::OpenAgentDiff"
     }