Accept `+` flag for opening at last line of file

[lmmx]

This was mentioned on the issue to introduce seeking to a given line, but I notice that the other standard behaviour, where no provided number with the + is interpreted as end of file is not used in helix:

Also of note is that + by itself is to open file and place cursor on last line of file.

Originally posted by @Paul-J-Barrett in #5437

I'm used to this and would like it in Helix too!