agent: Make the message editor expandable (#28420)

This PR allows expanding the message editor textarea to fit almost the
total height of the Agent Panel. Stylistically, I'm also changing the
font family we use in the textarea to use the buffer font; want to
experiment with this for a bit.

Release Notes:

- agent: The Agent Panel textarea can now be expanded to fill almost the
total height of the panel.

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

assets/keymaps/default-linux.json

@@ -631,6 +631,7 @@
       "ctrl-i": "agent::ToggleProfileSelector",
       "ctrl-alt-/": "assistant::ToggleModelSelector",
       "ctrl-shift-a": "agent::ToggleContextPicker",
+      "shift-escape": "agent::ExpandMessageEditor",
       "ctrl-e": "agent::ChatMode",
       "ctrl-alt-e": "agent::RemoveAllContext"
     }