Add examples

What's Changed

• Add sync.Map example by @devlights in #675
• Add sync.Once example by @devlights in #676
• Add Singleton example by @devlights in #677
• Bumps Go from 1.12.3 to 1.21.4 by @devlights in #678
• Add sync.Cond example by @devlights in #679
• Add sync.OnceFunc example by @devlights in #680
• Add sync.OnceValue example by @devlights in #681
• Add sync.OnceValues example by @devlights in #682

Full Changelog: v0.5.6...v0.5.7