Add Justfile

[notmandatory]

Describe the enhancement

Add a Justfile to make this project easier work-on and to use.

Use case

A few things a Justfile can do:

• cargo fmt, build, test, run
• start and stop regtest bitcoind for RPC testing

Additional context

Gist for Justfile that starts/stops and manages local regtest bitcoind:
https://gist.github.com/notmandatory/376c56f53f22c802ce34259636c9ed4b

Justfile for developing bdk_wallet:
https://github.com/bitcoindevkit/bdk_wallet/pull/248/files#diff-2f90408c2b0302b1cdb7f5d634114750837f940fa82d13057d9c18d0170a7e5c

[tvpeter]

fix by #198