[FEATURE] Add expression function to check if tag is valid (exists and not disabled)

[witman]

Yes please. The two properties we want to confirm for the same reason are exists and enabled. isGood is great, but doesn't help with this use case.

How to locate the number 'value' of a Tag's Data Quality

In addition to an expression for tag exists, an expression for isDisabled would be great. We use the same UDTs on a lot of different ancient and modern PLCs (thank you for tag property overrides and scripted tag creation!) and disable unused UDT members that don’t exist in the older PLCs. Templates adapt based on whether members are disabled or not. Unfortunately we seem to be stuck with magic strings to make this happen. Alternatively, we could use the isGood expression if it weren’t for the f…