[herald] Call slpv2 empp txs ALP instead of slp v2

Summary: New name for this token protocol. The formatting still needs to be improved, but this is pending indexing of ALP tokens. For now, they are labeled in a generic way that catches all EMPP txs.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15111