LUD-16: add optional default identifier shorthand

[andrerfneves]

Motivation

Lightning Address works well for hosted services, but personal domains often end up using placeholder local parts like me@domain, pay@domain, or ln@domain even when the domain itself is the intended identifier. This came up in andrerfneves/lightning-address#28.

Proposal

This extends LUD-16 with an entirely optional default identifier convention:

• services MAY expose _@domain as the canonical default identifier
• wallets MAY accept @domain as shorthand for _@domain
• resolution uses the existing LUD-16 endpoint shape: /.well-known/lnurlp/_
• wallets and services that do not implement the shorthand remain fully compatible

This avoids introducing a new endpoint shape and keeps metadata aligned with the existing LUD-16 user@site.com identifier format.

Testing

Docs-only spec change; no tests run.