Revert "lib: treat _ and - as part of a word"

This reverts commit 50dc4ab3.

The community has spoken:
- https://github.com/ohmyzsh/ohmyzsh/issues/9367
- https://github.com/ohmyzsh/ohmyzsh/issues/8743

Fixes #9367

lib/completion.zsh

 # fixme - the load process here seems a bit bizarre
 zmodload -i zsh/complist
 
-WORDCHARS='_-'
+WORDCHARS=''
 
 unsetopt menu_complete   # do not autoselect the first completion entry
 unsetopt flowcontrol