Issue 490 (#499)

* #490: Switched to shnex namespace for built-in node expression functions

* TTL file fixes

* Updated examples to use shnex

* Update shacl12-node-expr/index.html

Co-authored-by: simon <[email protected]>

* More sh:var -> shnex:var cleanup

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Co-authored-by: simon <[email protected]>

Author: HolgerKnublauch