Optionally propagate context to Decoder

[sethvargo]

This introduces a new way to define EnvDecode to optionally pass in a context. The old way still works, but the context method is preferred. Since it's actually impossible to define the same function with different arguments on the same receiver, there's no possibility of conflict.

Fixes #130

[Copilot]

Pull Request Overview

This PR introduces an enhanced decoder interface that optionally propagates the context to decoding functions while retaining legacy support for the old behavior.

• Updated the decoder interface to prefer context propagation via a new DecoderCtx interface while preserving the legacy Decoder interface.
• Updated functions (processAsDecoder, processField, etc.) to pass the context parameter.
• Updated tests and documentation (README.md) to reflect the new decoder changes.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
envconfig_test.go	Updated tests to validate both new DecoderCtx and legacy Decoder behaviors.
envconfig.go	Modified decoder functions to accept and propagate a context parameter; updated interface documentation.
README.md	Revised code snippet to demonstrate the new context-based custom decoder.

Comments suppressed due to low confidence (1)

envconfig_test.go:1444

• [nitpick] Test names should be unique to clearly differentiate scenarios; consider renaming one of the duplicate "syntax/=key" tests for clarity.

name: "syntax/=key",