explicitly forbid .js exports

[dynst]

A null target giving an explicit

Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './subpath/file.js' is not defined by "exports" in node_modules/cosmjs-types/package.json

is probably more understandable than the slightly inexplicable double-extension in

Error [ERR_MODULE_NOT_FOUND]: Cannot find module node_modules/cosmjs-types/subpath/file.js.js

https://nodejs.org/api/packages.html

[webmaster128]

Thanks!