Problem: diffs.nvim has no support for neojj, the neogit-like TUI for
jujutsu VCS. Users switching to jj get no syntax highlighting in neojj
status/diff buffers.
Solution: Mirror the existing neogit integration pattern for neojj.
Register `NeojjStatus`, `NeojjCommitView`, `NeojjDiffView` filetypes,
set `vim.b.neojj_disable_hunk_highlight` on attach, listen for
`User NeojjDiffLoaded` events, and detect jj repo root via
`neojj.lib.jj.repo.worktree_root`. Add parser patterns for neojj's
`added`, `updated`, `changed`, and `unmerged` filename labels.