Fix: Ensure fish completion handles multiline help strings correctly

[ReneSoltes]

This pull request addresses issue #3043 by correctly handling multiline help strings when generating shell completion scripts for the fish shell.

Problem:
Previously, commands with multiline help text would cause errors or incorrect completion behavior in fish shell due to improper escaping of newlines within the help strings.

Changes:

• Modified src/click/shell_completion.py to correctly escape newline characters (\n) within help texts before outputting them to the fish completion script.
• Added a new test case tests/test_shell_completion.py::test_fish_multiline_help_complete to specifically verify that multiline help strings are processed correctly and do not cause errors.
• The CHANGES.rst file has been updated to include this fix, referencing issue Multi-line help text is not handled correctly in fish #3043.
• The .gitignore file was also updated as part of the pre-commit hooks.

fixes #3043