Support ESM typescript modules

If your `tsconfig.json` has `"module": "ES2020"` or later (e.g you are outputting ESM and not CJS), TypeScript doesn't rewrite `import './foo'` to `import './foo.js'` in the transpiled output - instead you have to use `import './foo.js'` in your `.ts` file.

This module then breaks because it (quite reasonably) detects `'./foo.js'` as a dependency which of course doesn't exist.

I've hacked round it by replacing the file extension [here](https://github.com/dependents/detective-typescript/blob/main/index.js#L37) and [here](https://github.com/dependents/detective-typescript/blob/main/index.js#L45) but it doesn't take into account if `./foo.js` does actually exist and that's what you meant to import. I don't know the AST well enough to make that change, but a simple `fs.existsSync` should be enough?

Another workaround is to run the dep checker over the transpiled output instead of the ts source, but then you lose any `import type` declarations, so it's not ideal.

Refs: https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#type-in-package-json-and-new-extensions