feat: Add sequences defined by a formula in n to the main tool

  This is done by creating and exporting a SequenceFormula class.

  In addition, adding this class seemed to spark an additional cascade
  of the Vue components needing to be (more) strongly typed. So the commit
  has numerous new type annotations (that should have no effect other than
  to aid TypeScript type checking) and a slight modification/simplification
  to the interface between sequences.ts and visualizers.ts on the one hand
  and ToolMain.vue on the other, since I could not figure out out to type
  the old interface (and a lot of extraneous information that was never
  being used was being passed anyway). Hopefully these changes will
  eliminate issue numberscope#28.

  Also, again as part of shoring up the typing, finally eliminates the
  spurious additional copy of the p5 code in src/assets, instead relying
  on npm install to get the proper copy of p5 in the right place. (The point
  being that the idiosyncratic copy was not typed, but npm brings in the
  type definitions automatically if you rely on it.)

  Resolves numberscope#14.

package.json

@@ -11,6 +11,7 @@
     "axios": "^0.21.1",
     "core-js": "^3.6.4",
     "jquery": "^3.4.1",
+    "mathjs": "^9.5.1",
     "p5": "^1.0.0",
     "router": "^1.3.5",
     "vue": "^2.6.11",