Switch to manual generation of DTS files

[hossam-nasr]

Related to #352 and #416. Currently, the Durable SDK auto-generates its .d.ts from the source-code .ts files. We should switch to manually authoring our type declarations and our .d.ts files. This has multiple benefits:

• We can be intentional about which types we export, and what public APIs we support, helping us become more thoughtful about any breaking changes
• Helps us limit the number of unnecessary exports (see Limit amount of type (d.ts) files exposed to users #352), avoiding issues such as TypeScript build fails due to missing moment types #353.

[hossam-nasr]

Discussed some offline. As part of this, we should also try to switch from autogeneration of .d.ts files, to manual authoring of them. This should would help us become more intentional about which types we export, and also address #352. To do this, we should:

1. Move all types/interfaces that we should expose publicly to the types directory (in normal .ts files)
2. Separate class implementations from declarations, and move declarations to the types directory (also in .ts files)
3. Switch to manual .d.ts file generation

Work for points 2 and 3 is already being done

Originally posted by @hossam-nasr in #416 (comment)

[hossam-nasr]

Closed by #473