[Cashtab] [xecjs-message upgrade p1/3] Deprecate segwit support from xecjs-message

Summary:
T2784

Remove segwit support from the xecjs-message library. Test the async signing function in Cashtab to verify that segwit removal did not break any modified functions.

Publish this version to npm as a tagged dev release so that unit tests in Cashtab are not broken by local dependency.

Test Plan:
npm start
Sign a message
Confirm dev log displays 'Both methods not broken'
Verify a message
Confirm verification

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12645