Move WORDCHARS to the editor module?

I was looking for the definition of the WORDCHARS variable, and was surprised to see it in the completion module. It looks like it's been there since oh-my-zsh, but it seems like it belongs in the editor module rather than completion.

It's such a trivial thing it almost doesn't seem worth changing, but then again, as I said it was surprising to find it where it is now.

This pull request just moves the definition from completion/init.zsh to editor/init.zsh.