build(deps): bump underscore from 1.13.1 to 1.13.8 in /packages/core

Bumps [underscore](https://github.com/jashkenas/underscore) from 1.13.1 to 1.13.8.
- [Commits](https://github.com/jashkenas/underscore/compare/1.13.1...1.13.8)

---
updated-dependencies:
- dependency-name: underscore
  dependency-version: 1.13.8
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

packages/core/package.json

@@ -37,7 +37,7 @@
     "codemirror-formatting": "1.0.0",
     "html-entities": "~1.4.0",
     "promise-polyfill": "8.3.0",
-    "underscore": "1.13.1"
+    "underscore": "1.13.8"
   },
   "devDependencies": {
     "@types/markdown-it": "14.1.2",