Expose @sapper/app, @sapper/server, @sapper/common, @sapper/service-worker types to npm packages

[btakita]

It is useful to create npm packages that reference types defined in @sapper/app, @sapper/server, @sapper/common, @ @sapper/service-worker. However, these modules are defined within a sapper project in the src/node_modules directory, making the task of referencing these types from the npm package impossible (or not obvious).

It would be useful for a package author for these types to be defined in the sapper npm package and have documentation on how to reference these modules.