fix: use .ts instead of .d.ts

[johannschopplich]

Hi there! Thanks for creating this repo.

I was reading a Total TypeScript newsletter the other day that explains, why declaration files .d.ts should never be used. It's an anti-pattern, so to speak. Especially since you use:

useEmail/tsconfig.json

Line 8 in 355bb79

"skipLibCheck": true

Unfortunately, I can't find the newsletter online. But in a nutshell:

• skipLibCheck must always be set to true (since you don't control the types of installed Node modules). meaning .d.ts files are actually never type-checked.
  • Which was actually new to me. Imagine all the possible type issues that are just hidden.
• You can just use plain TypeScript files instead. They can do 99% of what .d.ts files can.

The only other resource I found was this article.

[fayazara]

LGTM