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

[nminchin]

For posterity, this was added into Paul's Ignition Extensions module:

Ignition Extensions - Convenience utilities for advanced users

Introducing a hopefully community-engaged effort to build a useful repository of scripting and other Ignition utilities for everyone to benefit from. Download the latest release from the Releases page and take advantage of the following functions: Scripting Functions system.util system.util.getContext(): Context A convenience function to retrieve the local "context" object, useful as an entrypoint to other advanced scripting. system.util.deepCopy(object): Object Converts a given Pyth…

It would still be nice for this to be added into the product, since this will be unusable in Edge projects