vim:ftplug:man: Enable resizing with hard-wrapping

From :h :Man:

> when running `man` from the shell and with that `MANPAGER` [='nvim
> +Man!'] in your environment, `man` will pre-format the manpage using
> `groff`. Thus, Nvim will inevitably display the manual page as it was
> passed to it from stdin. One of the caveats of this is that the width
> will _always_ be hard-wrapped

Since I actually don't like `g:man_hardwrap=0`/`MANPAGER=999` (e.g.
scrolling can be a mess with very long wrapped lines), add an
autocommand that is meant to reload the manpage through `:edit` after
every resize, so that its hard-wrapping adjusts to the new size.

This is slightly hacky, but does its job quite well.

Move the man.vim into after/ftplugin so that it overwrites the `set
wrap` of the global ftplugin, which I want turned off, since it messes
with the buffer shortly when resizing.

.config/vim/ftplugin/man.vim

@@ -1,7 +0,0 @@
-setlocal nospell
-
-" man(1) will assume it can use the full width of the terminal when
-" hard-wrapping the lines. When signcolumn is enabled the width is one cell
-" smaller and thus, lines that have a character in the last column will be
-" wrapped by vim (i.e. almost all of them).
-setlocal signcolumn=no