Fix justfile filetype detection in Neovim >= 0.10

[laniakea64]

Tag https://github.com/NoahTheDuke/vim-just/issues/89

This is draft because:

• We need to make sure this works when vim-just is loaded with lazy.nvim
• We should check if / how far the Neovim version cutoff can be lowered (within our supported range) without causing errors on older Neovim

[NoahTheDuke]

Sure, let's see how this goes.