[Cashtab] Add new helper function to generate to-spec targetOutputs for alias registration txs

Summary:
In order to fully deprecate legacy transaction building functions, need to also use the new sendXec function to broadcast alias registrations.

Currently this is done with an ad hoc custom function, registerNewAlias, in src/utils/transactions.js.

Follow existing logic for creating an OP_RETURN output. Implementation to broadcast alias txs will be in a separate diff.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15021