Formula grid (#542)

* wip: start FormulaGrid visualizer

* feat: Prototype FormulaGrid visualizer

* feat: prototype mouseover feature of FormulaGrid

* fix: correct aspect validation

* fix: interpret any number as a color in some way or other

* fix: Typescript doesn't know abs(number) is number??

* fix: Miscellaneous UI improvements

  * Put the help-info icon for a field near its top rather than its
    bottom so that it will not move as you resize a formula input area
  * Pop the resulting help info up above all other elements so the whole
    thing is readable
  * Validate and restore color codes correctly
  * At load time and whenever there is input, auto-grow formula text areas
    if necessary to show the entire formula

* feat: mouseover text opaque, shadowed, and moves left to right

* feat: inset is formula

* fix: edge browser z-indexing

* fix: updates per review

* doc: add diagram for hexagon and plug for using it with Triangle fill order

* featured specs

* doc: triangle diagram and misc fixes

* feat: mouseover popup travels with mouse pointer, avoiding in corner

* doc,test: document and test math.triangular and inverse

* fix,test: remove async race in Scope.vue mounting, update tests

* fix: Recompute size on visualizer reset(); e.g, aspect ratio might change

* ui: placeholder for aspect

* ui: ditto for dimensions

* feat: allow placeholder to remain on input, use for dimensions/aspect

* chore: briefer formulas in featured gallery and generate example images

* doc: final polish on new doc, some formula features for it

* chore: continuous integration snapshots

* doc: fix capitalization typo

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Co-authored-by: Kate Stange <kstange@math.colorado.edu>

Author: gwhitney