Add keybindings for accepting hunks (#19749)

I went with Cmd-Shift-Y on macOS (Ctrl-Shift-Y on Linux) for "yes accept
this individual hunk" - both are currently unused.

I went with Cmd-Shift-A on macOS (Ctrl-Alt-A on Linux) for "accept all
hunks" - both are unused. (Ctrl-Shift-A on Linux was taken, as is
Ctrl-Alt-Y, so although the pairing of Ctrl-Shift-Y and Ctrl-Alt-A isn't
necessarily obvious, the letters seem intuitive - "yes" and "all" - and
those key combinations don't conflict with anything.)

Release Notes:

- Added keybindings for applying hunks in Proposed Changes
<img width="247" alt="Screenshot 2024-10-25 at 12 47 00 PM"
src="https://github.com/user-attachments/assets/d6355621-ba80-4ee2-8918-b7239a4d29be">

assets/keymaps/default-linux.json

@@ -505,6 +505,13 @@
       "ctrl-enter": "assistant::InlineAssist"
     }
   },
+  {
+    "context": "ProposedChangesEditor",
+    "bindings": {
+      "ctrl-shift-y": "editor::ApplyDiffHunk",
+      "ctrl-alt-a": "editor::ApplyAllDiffHunks"
+    }
+  },
   {
     "context": "Editor && jupyter && !ContextEditor",
     "bindings": {