feat: add isDuration validator for ms-compatible duration strings

[Azarcodex]

Summary

Closes #2668

Adds a new isDuration(str, options?) validator that checks whether a string
is a valid duration in the format used by the popular
ms package.

This was originally implemented in typestack/class-validator
(issue,
PR), where maintainers
recommended it belong here in validator.js instead.

---

What isDuration validates

It accepts duration strings matching the ms package StringValue format:

Format	Example
Bare number (implicit ms)	'100', '3.5'
Number + short unit (no space)	'10s', '2m', '1h', '7d'
Number + short unit (with space)	'10 ms', '2 h', '5 w'
Number + long unit	'2 minutes', '3 hours', '4 days'
Decimal values	'1.5h', '0.5 days', '2.75 weeks'
Case-insensitive	'2 Days', '5 HOURS', '10MS'

Supported unit aliases

Unit	Accepted values
Years	years, year, yrs, yr, y
Months	months, month, mo
Weeks	weeks, week, w
Days	days, day, d
Hours	hours, hour, hrs, hr, h
Minutes	minutes, minute, mins, min, m
Seconds	seconds, second, secs, sec, s
Milliseconds	milliseconds, millisecond, msecs, msec, ms

---

Options

Option	Type	Default	Description
allowNegative	boolean	true	Whether negative durations like '-1h' are considered valid

---

Usage

// Valid
validator.isDuration('10s')                            // true
validator.isDuration('2 days')                         // true
validator.isDuration('1.5h')                           // true
validator.isDuration('100 milliseconds')               // true
validator.isDuration('100')                            // true (bare number = ms)
validator.isDuration('5 HOURS')                        // true (case-insensitive)

// Invalid
validator.isDuration('')                               // false
validator.isDuration('abc')                            // false
validator.isDuration('10xyz')                          // false (unknown unit)
validator.isDuration('1h 30m')                         // false (compound not supported)

// With options
validator.isDuration('-1s')                            // true  (default)
validator.isDuration('-1s', { allowNegative: false })  // false


## Tests

- ✅ 4 new test cases added in [test/validators/isDuration.test.js](cci:7://file:///c:/Users/vpaza/dev/validator.js/test/validators/isDuration.test.js:0:0-0:0)
- ✅ All 321 tests passing (`npm test`)
- ✅ Build and lint pass with 0 errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f39bb3b) to head (839c8a2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2674   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       115    +1     
  Lines         2592      2603   +11     
  Branches       660       665    +5     
=========================================
+ Hits          2592      2603   +11     

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[WikiRik]

The person that created the issue, also created a PR for it; #2669 . Feel free to review that one, but since this is the duplicate I will be closing this one