Type path for each export

[tomlarkworthy]

Fix for https://github.com/mhart/aws4fetch/issues/57

See https://github.com/microsoft/TypeScript/issues/52363

feature in npm docs is here https://www.typescriptlang.org/docs/handbook/esm-node.html#packagejson-exports-imports-and-self-referencing

Note "types" field must be first. Every export needs an entry. The existing root "types" is left as is as a fallback for old TS compilers.