JS refactor: We should replace all usage of `var x = ...` with `const` / `let` in the JS code

[rht]

[SangameshItagi]

Hi @rht , I can help here

[rht]

@SangameshItagi sounds good. Make sure to not do one big PR while doing it. It's better to do several small-medium sized PRs so that it is easier to review and merge.

[trickroll]

Tried to tackle a few of the files instead of a big PR!

[rht]

To avoid duplication of work:

In #1248, @SangameshItagi is working on

• mesa/visualization/templates/js/CanvasHexModule.js
• mesa/visualization/templates/js/CanvasModule.js
• mesa/visualization/templates/js/GridDraw.js
• mesa/visualization/templates/js/HexDraw.js

In #1246, @trickroll is working on

• mesa/visualization/templates/js/NetworkModule_sigma.js
• mesa/visualization/templates/js/PieChartModule.js
• mesa/visualization/templates/js/TextModule.js

[rht]

Remaining JS files up for grabs:

• BarChartModule.js
• ChartModule.js
• NetworkModule_d3.js
• runcontrol.js

[rht]

Closed via #1265.