Add bech32m mapping to browser field

[michalsmiarowski]

Closes https://github.com/bcoin-org/bcrypto/issues/66

lib/encoding/bech32m.js file is not properly replaced with lib/encoding/bech32m-browser.js when using the lib in the browser. The file should be mapped inbrowser field in package.json.

[pinheadmz]

Thanks! Sorry it took so long to test this, I'm running in to the same issue now, and this is the fix