Modernize cronexpr to Go 1.24

[Jason-Abbott]

• Create go.mod
• Update cronexpr.go
• Update cronexpr_next.go
• Update cronexpr_parse.go
• Update cronexpr_test.go
• Update example_test.go
• Update cronexpr/main.go
• Update .goreleaser.yaml
• Update READMEs
• Update CLAUDE.md
• Verify with go vet, go test, go test -race, go build

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