translation: rename zh_Han{s,t} to zh-{cn,tw}

Weblate created these previously missing files automatically, however it
follows a different naming style than used in the OpenWrt i18n system.

However, Weblate supports aliases which maps zh-{cn,tw} to it's
internally used zh_Han{s,t} codes, so files can be translated via
Weblate as before.

Signed-off-by: Paul Spooren <mail@aparcar.org>