Bug report: region_highlight converts `fg=default` to `none`, which is not the same

[Marlon Richert <marlon.richert@gmail.com>]

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Hi there, Zsh devs!

I am using Zsh 5.8 with `zsh-syntax-highlighting`. I want my paths to be
blue, but I want the slashes in each path to be my terminal's default text
color. I've configured `zsh-syntax-highlighting` to do so and it correctly
puts, for example,

```
region_highlight=( '0 2 fg=10' '3 16 fg=4' '3 4 fg=default' '9 10
fg=default' )
```

where `3 16` is the path I'm trying to `cd` to and  `3 4` plus `9 10` are
the positions of slashes in the path.

However, after this assignment occurs, when I `print -r
"${(q+)region_highlight[@]}"`, I get as output

```
'0 2 fg=10' '3 16 fg=4' '3 4 none' '9 10 none'
```

and when the ZLE highlights the line, the slashes are rendered in the same
blue color as the rest of the path. If I use any other `fg` value than `
default`, then the slashes are colored correctly.

This seems like incorrect behavior to me, on two accounts:

 1. It seems incorrect to convert `fg=default` to `none`. From reading the
[documentation](
http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#Character-Highlighting),
`none` is not supposed to be the same as `fg=default`.

 2. It also seems incorrect to me that `none` effectively does nothing at
all. According to the documentation, `none` should mean that

    > No highlighting is applied to the given context.

    Instead, `none` just appears to do nothing at all, which seems useless
to me; if I don't want to change the highlighting of that part of the line,
then I can just not add a spec for it.

Do you agree and could someone be so kind as to fix this? :)

Cheers,

Marlon

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