debugger: Improve keyboard navigability of variable list (#31462)

This PR adds actions for copying variable names and values and editing
variable values from the variable list. Previously these were only
accessible using the mouse. It also fills in keybindings for expanding
and collapsing entries on Linux that we already had on macOS.

Release Notes:

- Debugger Beta: Added the `variable_list::EditVariable`,
`variable_list::CopyVariableName`, and
`variable_list::CopyVariableValue` actions and default keybindings.

assets/keymaps/default-linux.json

@@ -873,6 +873,16 @@
       "ctrl-i": "debugger::ToggleSessionPicker"
     }
   },
+  {
+    "context": "VariableList",
+    "bindings": {
+      "left": "variable_list::CollapseSelectedEntry",
+      "right": "variable_list::ExpandSelectedEntry",
+      "enter": "variable_list::EditVariable",
+      "ctrl-c": "variable_list::CopyVariableValue",
+      "ctrl-alt-c": "variable_list::CopyVariableName"
+    }
+  },
   {
     "context": "BreakpointList",
     "bindings": {