fix: create app template error

[alexzhang1030]

Here is the thing:

• package create-app invoke build
• build command copy files

but ../../demo/starter/slides.md includes ../../demo/starter/pages/multiple-entries.md:

So we must copy multiple-entries.md when build.

I create a script to do this work.

Hope will fix this issue :)

[dtinth]

Seems like the fix is still unreleased, so creating a slidev today still results in an error.