Only append to the highest function ID in registry.yml

[kbenzie]

As noted in #584 (comment) when the sequence of enumerations of function ID's in scripts/core/registry.yml contains a gap, the script will fill that gap.

XPTI expects function ID's to be globally unique over time, so reusing ID's of functions which have been removed is not ideal. Instead we should only incremenet the highest ID when adding new functions.