ThemeSet.pack's before option breaks @charset and @import at-rules

[yhatt]

ThemeSet.pack can inject string before and after packed CSS. By setting before option, theme's @charset and @import at-rules would not work because of the specification of CSS.

It means that marp-team/marp-core, is using before option in ThemeSet.pack, cannot import another theme by @import rule. We require roll-up at-rules before parsing @import for theme.