ProhibitAutocmdWithNoGroup: Ignore for buffer-specific autocommands

[somini]

This rule should be relaxed to allow for defining autocmd outside augroup, as long as the <buffer> option is set.

This is used for example to create filetype-specific autocommands by placing them in a filetype/&filetype/commands.vim file or similar.

[somini]

See https://github.com/Kuniwak/vint/blob/172b1105b288ac0c85448534d520213d24fe78d3/vint/linting/policy/prohibit_autocmd_with_no_group.py#L35-L42

[natebosch]

It should also allow with ++once.