While this is not the answer you would like to hear, tsconfig.json marks a directory as the root of a typescript project, just like the package.json marks a directory as the root of a node.js project. These files help tools like IDEs to understand your project.
In my opinion you should only consider moving tsconfig.json when you start to put your package.json in some other folder as well.
See also: https://www.typescriptlang.org/docs/handbook/tsconfig-json.html