Longturtle improvements

[nicholascar]

Summary of changes

Fixes some ugliness in the longturtle format, in particular:

• double newline at the end of the file
• inconsistent Blank Node bracket positions - some line training, some newlining, now all newline
• Blank Node incorrect indentation - now they are all 1 indentation further right than their parents and their content is another further indent right

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • N/A
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.924% (-0.001%) from 90.925% when pulling 476c97b on longturtle_improve into b1ff089 on main.

[aucampia]

pre-commit.ci autofix

[aucampia]

Change looks good