chore: update vexflow to 1.2.87

adds directionless arpeggio, removes npm audit vulnerabilities from vexflow
change percussion note head, add square, rectangle note heads
see vexflow issue #660 for changelog

package.json

@@ -54,7 +54,7 @@
     "loglevel": "^1.5.0",
     "shortid": "^2.2.6",
     "typescript-collections": "^1.1.2",
-    "vexflow": "^1.2.85"
+    "vexflow": "^1.2.87"
   },
   "devDependencies": {
     "@types/chai": "^4.0.3",