These are TypeScript definitions for REPENTOGON, an exe-hack for the modding API of The Binding of Isaac: Repentance.
They are not included by default in IsaacScript mods.
First, add these definitions as a dependency (similar to any other JavaScript/TypeScript dependency):
npm install isaac-typescript-definitions-repentogon --save
Second, add the following to your "tsconfig.json" file:
"compilerOptions": {
"types": ["isaac-typescript-definitions-repentogon"],
},