Add unmarshaler interface

[sagikazarmark]

Based on the PRs below. I opted for creating a new PR, because some of the edge cases weren't handled and tests were AI slob.

Fixes #11
Fixes #155
Fixes #164

[Copilot]

Pull request overview

This pull request adds an Unmarshaler interface to mapstructure, allowing types to define custom unmarshaling behavior similar to json.Unmarshaler from the standard library. This is a follow-up to previous PRs (#11, #155, #164) with improved edge case handling and test quality.

Changes:

• Added Unmarshaler interface with UnmarshalMapstructure(any) error method
• Integrated Unmarshaler check into the decode flow (runs after DecodeHook but before standard decoding)
• Added DisableUnmarshaler config option to opt out of Unmarshaler behavior
• Comprehensive test coverage including nested structs, embedding, non-struct types, error handling, and interaction with DecodeHook
• Added example demonstrating default value application via Unmarshaler

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
mapstructure.go	Core implementation: defines Unmarshaler interface, adds DisableUnmarshaler config option, implements getUnmarshaler helper, and integrates Unmarshaler check into decode flow
mapstructure_test.go	Comprehensive test suite covering basic usage, DecodeHook interaction, disable flag, nested structs, embedding, error handling, pointer receivers, non-struct types (string, slice, map), metadata, and struct-to-map decoding
mapstructure_examples_test.go	Example demonstrating Unmarshaler interface usage for applying default values

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