feat: improve utility of isAddress

[zzmp]

Improves the utility of isAddress:

• allows it to check any type (unknown), instead of restricting it to string
• returns string | false, so that an unknown type passed in can then be narrowed to a string without coercion

This will not break usages in conditionals, as string is truthy, so string | false can still be used eg in if (isAddress(addr)). This will break usages where true/false are explicitly checked, such as isAddress(addr) === true/!== false.