GroupItemOptions and ToggleNode should know what subtype of Node they are creating

[samreid]

For phetsims/center-and-variability#170, it would be helpful for GroupItemOptions and ToggleNode to know what subtype of Node they are creating.

[samreid]

@zepumph can you please help review this issue? See also TODOs in phetsims/center-and-variability#201 where type inference isn't working as expected. This issue is related to iteration item https://github.com/orgs/phetsims/projects/65?pane=issue&itemId=27371596 which is priority 10.1 overall.

[zepumph]

We got rid of the defaults and type inference is working well when each createNode creates the same type.

When having the heterogenous elements list like in VariabilityPlotNode, we found microsoft/TypeScript#33664 and microsoft/TypeScript#31617 (comment) and as a result are happy to chock this up to a weirdness of TypeScript lazily inferring the first generic. Typescript has done this by design, and they aren't changing.

We are ready to close and are fine specifying things manually when needing a Supertype for a heterogenous array of elements.