move withInlineVoid to core

[lukesmurray]

The code in this file is useful even outside of using the plugins.

https://github.com/udecode/slate-plugins/blob/next/packages/slate-plugins/src/common/plugins/inline-void/withInlineVoid.ts

Without it the inlineTypes and voidTypes properties on the plugin interface don't do much.
Also would be helpful to add some sort of comment to inlineTypes and voidTypes to indicate that you need to use withInlineVoid in order to get them to work.