Add types export to package.json

[pdubs10]

I needed to add this export to package.json in order get the library working properly with TypeScript 5.1.3 and a simple Vue application based off of npm init [email protected].

Without this change VSCode gives the following error when attempting to import the module.

Could not find a declaration file for module 'aws4fetch'. '/path/to/node_modules/aws4fetch/dist/aws4fetch.esm.mjs' implicitly has an 'any' type.
  There are types at '/path/to/node_modules/aws4fetch/dist/main.d.ts', but this result could not be resolved when respecting package.json "exports". The 'aws4fetch' library may need to update its package.json or typings.ts(7016)

[DSergiu]

@mhart any plans on merging this? Much appreciate it.