feat: refactor API for CustomLexer

[ianlewis]

Description:

Refactor the API for CustomLexer to add a CustomLexerContext. The context wraps the context.Context during lexing, and provides methods for accessing the input stream.

Related Issues:

Updates #173

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Update documentation if applicable.
• Add a note in the CHANGELOG.md if applicable.

[Copilot]

Pull request overview

This PR refactors the CustomLexer API by introducing a new CustomLexerContext type that wraps context.Context and provides access to lexer methods. The refactoring moves lexer interaction methods from CustomLexer to CustomLexerContext, changing the signature of LexState.Run to accept the new context type instead of both context.Context and *CustomLexer.

Changes:

• Introduced CustomLexerContext type that embeds context.Context and provides methods for lexer interaction
• Updated LexState.Run signature to accept *CustomLexerContext instead of context.Context and *CustomLexer
• Moved public lexer methods (Peek, Advance, Emit, etc.) to CustomLexerContext while keeping implementation in CustomLexer as private methods
• Updated all lexer state functions and tests to use the new API
• Updated tests to use t.Context() instead of manually creating contexts with cancel functions

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
custom.go	Added CustomLexerContext type with public methods delegating to private CustomLexer methods; updated LexState interface
custom_test.go	Updated all test functions to use CustomLexerContext instead of direct CustomLexer access; renamed tests to reflect context usage
template_example_test.go	Updated lexer state functions to accept *CustomLexerContext parameter
ini_example_test.go	Updated lexer state functions to accept *CustomLexerContext parameter
lexparse_test.go	Updated test contexts to use t.Context() and updated lexErrState.Run signature
CHANGELOG.md	Added entry documenting the API refactoring

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