.d.ts files should be forbidden from importing .ts files

[sandersn]

.d.ts files should not be able to import .ts files; .ts files are supposed to be only for testing. But it's too easy to make a mistake and use .ts for a file intended for shipping types. See for example https://discord.com/channels/508357248330760243/757992230077333655/1086956594736148511

To repro:

cd $dt/types/adobe__pdfservices-node-sdk
git mv client-config.d.ts client-config.ts
npm test adobe__pdfservices-node-sdk

Expected: Error "Index.d.ts can't import from client-config.ts because it's a .ts file"
Actual: Compile error because client-config.ts uses syntax that is illegal in .ts (function signature without declare or following implementation signature).