feat: bundle type definitions of dev-deps

[brc-dd]

fixes #1254

[brc-dd]

There are some more changes.

• We need to manually define Header because shared.d.ts is not processed by rollup (hence, types from @mdit-vue/plugin-header won't be bundled).

• shared.d.ts was getting bundled (and duplicated) in client and node types, so externalized that.

• Explicit re-exporting is now not required in types/index.d.ts because exports now don't have any conflicts.

• Stuff like this can now again be done without any issues:

  declare module 'vitepress' {
    interface Header {
      compositionOnly?: boolean
      optionsOnly?: boolean
    }
  }

[meteorlxy]

LGTM