Expand terminal menu actions (#14828)

<img width="422" alt="image"
src="https://github.com/user-attachments/assets/73195894-81a7-4b8e-b5cf-ae60bf5b1fb9">

Release Notes:

- Added `Copy`, `Paste`, `Select All`, & `New Terminal` into terminal's context menu

assets/keymaps/default-linux.json

@@ -606,6 +606,7 @@
       "ctrl-alt-space": "terminal::ShowCharacterPalette",
       "shift-ctrl-c": "terminal::Copy",
       "ctrl-insert": "terminal::Copy",
+      "ctrl-a": "editor::SelectAll",
       "shift-ctrl-v": "terminal::Paste",
       "shift-insert": "terminal::Paste",
       "ctrl-enter": "assistant::InlineAssist",

assets/keymaps/default-macos.json

@@ -628,6 +628,7 @@
       "ctrl-cmd-space": "terminal::ShowCharacterPalette",
       "cmd-c": "terminal::Copy",
       "cmd-v": "terminal::Paste",
+      "cmd-a": "editor::SelectAll",
       "cmd-k": "terminal::Clear",
       "ctrl-enter": "assistant::InlineAssist",
       // Some nice conveniences