Make Xpub Scan more easily configurable

[glethuillier]

The configuration options are set in src/settings.ts.

Therefore, each time an end-user configures the tool, it has to be rebuilt.

A more user-friendly approach could be to make it a JSON (or, preferably, a YAML) file automatically parsed (and validated) when the user runs the tool.

[twilgenbus-ledger]

I think you've recently added the library dot-env to the project? Using the .env could also be a clean way to set this configuration.

[glethuillier]

@twilgenbus-ledger That could be a great idea indeed. Note that we also need to provide a default configuration: the tool should be operational even without any .env.