[Cashtab] Update bip21 parsing to match latest spec, op_return_raw vs opreturn

Summary:
Since electrum already has support for it, we are using the param name op_return_raw instead of opreturn. This has already been updated in doc/standards.

This isn't implemented yet on any Cashtab screens, so only need to update the function + unit tests.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15206