feat: enable declarationMap, use .d.ts files in monorepo too

Configure TSC to emit declaration maps, providing information needed by
IDEs like VSCode to jump from usage of a symbol to its definition in
the original source code.

Simplify layout of all packages, remove top-level `index.ts` and
`index.d.ts` files in favor of a package.json entry

    "types": "dist/index.d.ts"

Simplify layout of module packages (excluding example applications) and
remove the top-level `index.js` file in favor of a package.json entry
    "main": "dist/index.js"

Update developer documentation and project templates accordingly.

Benefits of these changes:

- It's a preparation step for TypeScript Project References

- Build is consuming less CPU cycles now. User time used to be 5m21
  before, it's 1m48s now. Real time went down from 0m49s to 0m37s.

- Simpler project setup in our packages, we no longer need to maintain
  dummy index files at the top level.

Drawbacks:

- Refactor-rename does not work across packagas. I believe this is a
  limitation of TypeScript, hopefully it will be eventually fixed.

- Navigation across packages may not work until the monorepo is built.

Author: bajtos

benchmark/index.ts

@@ -9,6 +9,7 @@
     "benchmark"
   ],
   "main": "index.js",
+  "types:": "dist/index.d.ts",
   "engines": {
     "node": ">=8.9"
   },

benchmark/package.json

@@ -10,6 +10,7 @@
     "express"
   ],
   "main": "index.js",
+  "types": "dist/index.d.ts",
   "engines": {
     "node": ">=8.9"
   },

docs/site/DEVELOPING.md

@@ -3,6 +3,7 @@
   "version": "1.1.20",
   "description": "A simple hello-world Application using LoopBack 4",
   "main": "index.js",
+  "types": "dist/index.d.ts",
   "engines": {
     "node": ">=8.9"
   },