Ignore .jj

[offbyone]

Works around pypa/hatch#304 which is a broad problem with .ignore files in Hatchling

What's truly funny is that it's only a blocker to create an sdist; the files that are scanned won't end up there, but building the sdist fails due to

Using Python 3.12.14 environment at: .tox/typing
  × Failed to build `pyhamcrest @
  │ file:///Users/offby1/projects/PyHamcrest/.tox/.tmp/package/131/pyhamcrest-2.1.1.dev45.tar.gz`
  ├─▶ Failed to extract archive: /Users/offby1/.cache/uv/sdists-v9/.tmppen6ft
  ├─▶ Invalid tar file
  ├─▶ failed to unpack `/Users/offby1/.cache/uv/sdists-v9/.tmppen6ft/pyhamcrest-2.1.1.dev45/.jj/repo/config.toml`
  ╰─▶ symlink path `/Users/offby1/.config/jj/repos/b898e8b230458c422d8b/config.toml` is absolute, but external
      symlinks are not allowed
typing: exit 1 (0.06 seconds) /Users/offby1/projects/PyHamcrest> /Users/offby1/.local/share/uv/tools/tox/bin/uv pip install --reinstall --no-deps pyhamcrest@/Users/offby1/projects/PyHamcrest/.tox/.tmp/package/131/pyhamcrest-2.1.1.dev45.tar.gz pid=90473
typing: FAIL ✖ in 1 minute 19.07 seconds