feat: introduce setMode() helper for lib legacy/strict mode switching

[antongolub]

Relates

• isV6Format returns true with IPv4 input indutny/node-ip#135
• ip.isV4Format('999.999.999.999') returns true indutny/node-ip#159
• Not validating properly (IPV4 with block >= 256 still returns true) indutny/node-ip#105

import ip from '@wepbod/ip'

ip.isV4Format('255.255.255.256')   // true
ip.isV6Format('127.0.0.1')         // true

ip.setMode('strict')               // or 'legacy'
ip.isV4Format('255.255.255.256')   // false
ip.isV6Format('127.0.0.1')         // false

// new methods are always strict
ip.isV4('255.255.255.256')         // false
ip.isV6('127.0.0.1')               // false