fix: do not return EOF from Err

[ianlewis]

Description:

Do not return io.EOF from CustomLexer.Err. Err should only return unexpected errors.

Related Issues:

Fixes #170

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 fixes the CustomLexer.Err() method to correctly return nil at EOF instead of io.EOF. The change aligns with the expected behavior that Err() should only return unexpected errors, not the normal end-of-file condition.

Changes:

• Removed the assignment of io.EOF to l.err when reaching end of input
• Updated test to expect nil instead of io.EOF from Err()
• Added documentation comment for the newToken method

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
custom.go	Removed io.EOF assignment when reaching end of input and added documentation for newToken method
custom_test.go	Updated test expectation to verify Err() returns nil at EOF

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