Add `mkfs.*` completion specs

[grant0417]

Sanity checks

• [X] My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• [X] I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• [ ] I would like to work on this.

What CLI tool does this relate to?

mkfs.*

Which statement makes the most sense?

This completion spec does not exist

Issue Details

/bin/mkfs.bfs	 /bin/mkfs.cramfs  /bin/mkfs.ext2  /bin/mkfs.ext4  /bin/mkfs.minix  /bin/mkfs.ntfs  /bin/mkfs.xfs
/bin/mkfs.btrfs  /bin/mkfs.exfat   /bin/mkfs.ext3  /bin/mkfs.fat   /bin/mkfs.msdos  /bin/mkfs.vfat