message: Add sum snippet
diff: "body": "\\int_{$1}^{$2}$0",
  "description": "Insert an integral"
  },
  + "sum": {
  + "prefix": "@A",
  + "body": "\\sum_{$1}^{$2}$0",
  + "description": "Insert an sum"
  + },
  "Big|": {
  "prefix": "@|",
  "body": "\\Big|",
  