Change "types" to "types/index.d.ts" in package.json

[AaronBeaudoin]

Basically, the line in package.json where the types path is specified does not align with TypeScript's official documentation for publishing because it points to a directory and not a file. This cases VS Code to display the following error in the standard jsconfig.json file at the root of a project's directory:

See this StackOverflow thread for more information.

[josephfrazier]

Thanks for the report, @AaronBeaudoin, and for linking the stackoverflow thread! Would you mind opening a PR with the desired change?