Add the Var and FlagSet variants for completeness

[ldemailly]

And better doc/examples

Should then be good for 1.0.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0cd677c) to head (7f4d36b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          174       182    +8     
=========================================
+ Hits           174       182    +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull Request Overview

This PR adds FlagVar and FlagSet variants of the duration flag functions to complete the API parity with Go's standard flag package. The changes prepare the package for a 1.0.0 release by providing all the flag registration variants that users would expect.

• Adds FlagVar and FlagSetVar functions for binding duration flags to existing variables
• Adds FlagSet function for registering duration flags on specific flag sets
• Refactors existing Flag function to use the new FlagSet implementation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
duration.go	Implements new flag variants and refactors existing Flag function to use FlagSet
duration_test.go	Adds test coverage for the new FlagVar functionality

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}