Emit type declarations

[omkar-ethz]

Motivation

Currently, the ro-crate package as published on npm does not include type declarations. This means ts projects cannot benefit from typings for ROCrate, Entity, etc.
This PR attempts to fix project configuration to export type declarations in the npm package.

Changes

• Add declaration: true to tsconfig
• Rename types.d.ts to types.ts, as .d.ts files are not considered for compilation (stackoverflow)
• include type declarations in npm package by adding types to package.json

[omkar-ethz]

Tested with my fork that the published npm package includes type declarations: https://www.npmjs.com/package/@omkar.xyz/ro-crate