`__ISVALIDREF` helper function

[volsa]

Is your feature request related to a problem? Please describe.
#1251 introduced the REF= and REFERENCE TO (extension) keywords. According to CodeSys the __ISVALIDREF function is also part of that extension however and currently missing.

Describe the solution you'd like
Implement the given function either as part of the stdlib or as a builtin (probably the latter though)