Allow insertion into array above / below existing items

[fsteger]

Reasons for making this change

Add function that allows adding new items anywhere inside the array. New function is available on each array item as onAddIndexClick

#1073

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

Looks good - thanks!

[gilomen2]

@epicfaace do you know which release this is planned to be apart of and when that release might happen? this is a super useful feature.

[epicfaace]

@gilomen2 trying to get a release out soon; honestly just trying to find a fix for #1396 because if we release the code as-is, it will break some functionality.