Bump acorn from 6.0.5 to 6.4.1 in /_tools

Bumps [acorn](https://github.com/acornjs/acorn) from 6.0.5 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/compare/6.0.5...6.4.1)

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

_tools/package-lock.json

@@ -10,9 +10,9 @@
       "integrity": "sha512-sY5AXXVZv4Y1VACTtR11UJCPHHudgY5i26Qj5TypE6DKlIApbwb5uqhXcJ5UUGbvZNRh7EeIoW+LrJumBsKp7w=="
     },
     "acorn": {
-      "version": "6.0.5",
-      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.0.5.tgz",
-      "integrity": "sha512-i33Zgp3XWtmZBMNvCr4azvOFeWVw1Rk6p3hfi3LUDvIFraOMywb1kAtrbi+med14m4Xfpqm3zRZMT+c0FNE7kg=="
+      "version": "6.4.1",
+      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.4.1.tgz",
+      "integrity": "sha512-ZVA9k326Nwrj3Cj9jlh3wGFutC2ZornPNARZwsNYqQYgN0EsV2d53w5RN/co65Ohn4sUAUtb1rSUAOD6XN9idA=="
     },
     "acorn-globals": {
       "version": "4.3.0",