Arcade Editor/Map Spec: Add Map Level User-Defined Functions

[JustinReynolds]

Add the ability to create User-Defined functions at the map level that can be called by any expression like Arcade's built-in functions. This will dramatically cleanup a lot of reused code from one field or layer to the next. Write it once, call it when it is needed.

What might be needed to make this happen.

• A place in Field Maps to author these functions.  
• A new section in the maps definition (JSON)... e.g. userDefinedFunctionInfos[]
• Define function properties:
  • function name
  • function expression
  • function description (shown in the blue information symbol, so a user knows what the function does and what parameters it takes)
• A new tab in the Arcade Editor "User-Defined Functions".
• Validating that the name of the function does not conflict with the name of a built-in function or prefixing the name of the function with text that ensures it can never conflict with a built in function.

[JCGuarneri]

This would be really useful to us. We have a bit of code that gets hyperlinks for as-builts from intersecting areas. Right now, we need to apply this code individually to each layer where we want to see those links. If I need to make a change, I need to change that in every layer to make sure things work properly.