fix(isFQDN): skip numeric-label check when require_tld is false

[xxiaoxiong]

Problem

When require_tld: false, single-label names that are all-numeric (e.g. 192, 10) are incorrectly rejected by isFQDN.

isFQDN('192', { require_tld: false })  // returns false  ❌ (should be true)
isFQDN('10',  { require_tld: false })  // returns false  ❌ (should be true)

Root cause: The check at line 47 (outside the require_tld block) was unconditionally rejecting any label whose last dot-segment consists solely of digits:

// before
if (!options.allow_numeric_tld && /^\d+$/.test(tld)) {
  return false;
}

When require_tld: true this is fine — TLDs like .123 are always disallowed. But when require_tld: false there is no TLD at all; the last segment is just the only label, and a purely numeric label should be permitted.

The same constraint is already enforced inside the require_tld block (line 36 rejects non-alphabetic TLDs), so the outer check is redundant for the require_tld: true case and harmful for the require_tld: false case.

Fix

Guard the check with options.require_tld:

// after
if (options.require_tld && !options.allow_numeric_tld && /^\d+$/.test(tld)) {
  return false;
}

Tests

Added test cases for { require_tld: false } in test/validators/isFQDN.test.js covering:

• Standard multi-label FQDNs (google.com) still pass
• Non-numeric single labels (localhost) pass
• Numeric single labels (192, 10) now correctly pass

Fixes #2425