Use eslint instead of jshint in Sandcastle

[mramato]

The Sandcastle editor uses a separate set of jshint rules than the eslint rules now being used by Cesium. We should switch Sandcastle over to running eslint in the browser so we can get rid of jsHint completely.

It looks like ESLint currently doesn't work in the browser out of the box, but it appears easily fixable. See this issue for details: eslint/eslint#8348

[thw0rted]

Came here to open a new issue asking for better Sandcastle style checking, and found this. Is it anywhere on the roadmap? Would you consider a more full-featured in-browser editor like maybe https://github.com/microsoft/monaco-editor ? Every time I write a const or let or arrow function in a Sandcastle example it highlights the line as invalid, because it doesn't speak modern ECMAScript.