feat: move custom variables to $(custom:...) with backward compatibility

[krocheck]

Responds, in part to #2812 by moving custom variables from $(internal:custom_...) to $(custom:...). Checks are added to maintain backwards compatibility with legacy names.

If we want to move forward I'll get the rest of the action items executed.

My main reason for attempting this is that with the addition of $(this:...), it seems logical to me to make this change. The fact that custom isn't a reserved word in v2.x is concerning, but I think is quite the edge case for someone to completely rename an instance to custom. But that can be investigated for upgrade mitigation ... though scrubbing the variables might become difficult.

TODO:

• [x] Validate basic functionality and backwards compatibility
• [ ] Validate custom variable internal actions
• [ ] Validate triggers
• [ ] Update documentation
• [ ] Update any labels, like indicated in #2812
• [ ] Investigate custom instance name mitigation in v2-v3 upgrade