refactor: replace interface{} with any (Go 1.18+)

[MD-Mushfiqur123]

Replace all occurrences of interface{} with any (available since Go 1.18). This is a mechanical refactor with no behavioral changes.

Files changed:

• equal.go
• config.go
• constructors.go
• option.go
• static_test.go
• config_ext_test.go
• option_test.go
• internal/merge/merge.go
• internal/merge/merge_test.go

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