Make `_zsh_highlight-zle-buffer` just a hands-off.
Now, `_zsh_highlight-zle-buffer` calls each ZSH_HIGHLIGHT_FUNCTIONS
in turn. The former `_zsh_highlight-zle-buffer` is renamed to
`_zsh_main-highlight.`
Signed-off-by: Takeshi Banse <takebi@laafc.net>