feat: Add cds-dk to default export

[daogrady]

We have to remove the export of cds-dk before performing the rollup, as API extractor does not understand the export * syntax. When adding it back in, it was only added to the named exports so far. This PR adds it to the default export as well.

[daogrady]

Closing, as export declarations are not permitted in namespaces.

We therefore can not do

declare namespace '@sap/cds' {
  export * from '@sap/cds-dk';
}