Add a minimal ftplugin `runtime/ftplugin/yara.vim` that sets:
- `commentstring` for YARA line comments (`//`)
- `comments` for YARA block comment (`/* */`)
- `formatoptions` to wrap comment lines and continue comment after newlines
This was heavily inspired from `runtime/ftplugin/c.vim`
closes: vim/vim#1973639ee7d17b9
Co-authored-by: Thomas Dupuy <thom4s.d@gmail.com>