feat: update CustomLexerContext to CustomLexerCursor

[ianlewis]

Description:

Rename CustomLexerContext to CustomLexerCursor and separate it from the context.Context.

Add a NewCustomLexerCursor function to make it easier to create a cursor for testing lexer functions.

Related Issues:

• [feature] improve testability #188

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 updates the CustomLexer state-machine API by renaming CustomLexerContext to CustomLexerCursor and passing context.Context separately, improving ergonomics for testing lexer state functions (per #188).

Changes:

• Rename CustomLexerContext → CustomLexerCursor and update LexState.Run/LexStateFn to accept (context.Context, *CustomLexerCursor).
• Add NewCustomLexerCursor(*CustomLexer) helper and update CustomLexer.NextToken to use it.
• Update README and example/tests to the new cursor-based API.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
custom.go	Introduces CustomLexerCursor, updates LexState API and NextToken to pass context.Context separately.
custom_test.go	Updates unit tests to use CustomLexerCursor and the new state signature.
lexparse_test.go	Updates a lexer test state to match new LexState.Run signature and imports context.
template_example_test.go	Updates template lexer example states to accept (context.Context, *CustomLexerCursor).
ini_example_test.go	Updates INI lexer example states to accept (context.Context, *CustomLexerCursor).
README.md	Updates public documentation/examples for the renamed cursor and new Run signature.

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