Describe alternatives to require.extensions

https://nodejs.org/api/modules.html#modules_require_extensions says `require.extensions` is deprecated. Per https://github.com/TypeStrong/ts-node/issues/641 and https://github.com/TypeStrong/ts-node/issues/989 there are no alternatives. They reference further the documentation of @babel/node, @babel/register, and coffeescript. • Either re-introduce require.extensions or describe more precisely the alternatiles that should be used.