Reenable TypeScript checking of libraries

[smarr]

In #230, I added "skipLibCheck": true to the tsconfig.json, which disables the type checking of library code.

This was done because I got the following errors, and didn't find a solution:

node_modules/@types/express-serve-static-core/index.d.ts:395:18 - error TS2430: Interface 'Request<P, ResBody, ReqBody, ReqQuery, LocalsObj>' incorrectly extends interface 'IncomingMessage'.
  Types of property 'body' are incompatible.
    Type 'ReqBody' is not assignable to type 'string | { [key: string]: unknown; } | undefined'.
      Type 'ReqBody' is not assignable to type '{ [key: string]: unknown; }'.

395 export interface Request<
                     ~~~~~~~

  node_modules/@types/express-serve-static-core/index.d.ts:398:5
    398     ReqBody = any,
            ~~~~~~~~~~~~~
    This type parameter might need an `extends { [key: string]: unknown; }` constraint.
  node_modules/@types/express-serve-static-core/index.d.ts:398:5
    398     ReqBody = any,
            ~~~~~~~~~~~~~
    This type parameter might need an `extends string | { [key: string]: unknown; } | undefined` constraint.

This should be revisited, and skipLibCheck should be removed again.