control-[ to escape, control-m to return

karabiner.json

{
  "description": "control-[ to escape",
  "manipulators": [
    {
      "type": "basic",
      "from": {
        "key_code": "open_bracket",
        "modifiers": { "mandatory": [ "control" ] }
      },
      "to": [
        { "key_code": "escape" }
      ]
    }
  ]
},
{
  "description": "control-m to return",
  "manipulators": [
    {
      "type": "basic",
      "from": {
        "key_code": "m",
        "modifiers": { "mandatory": [ "control" ] }
      },
      "to": [
        { "key_code": "return_or_enter" }
      ]
    }
  ]
}