Add missing vsce dependency in package.json.

Author: olafurpg

package.json

@@ -47,7 +47,8 @@
         "github-changes": "^1.1.2",
         "ts-node": "^8.1.0",
         "tsc": "^1.20150623.0",
-        "typescript": "^3.3.3"
+        "typescript": "^3.3.3",
+        "vsce": "^1.59.0"
     },
     "scripts": {
         "vscode:prepublish": "test -f ./syntaxes/Scala.tmLanguage.json",