Use shared type tests for preview and stable types

This guarantees that we will be testing exactly what our consumers use,
with the only difference being the specific import paths used to expose
the side effect modules. It also keeps the type tests from diverging
between the two sets of types.

Previously, following the DefinitelyTyped approach, all the preview
type modules had their own `tsconfig.json`, but this actually made them
fail to interoperate with stable types, and this approach *only* works
if they work together. Removing those means they operate as a single
coherent set of type definitions with the stable types, distinguished
*only* by the way they are authored.

Author: chriskrycho

package.json

@@ -50,9 +50,8 @@
     "test:blueprints": "yarn test:blueprints:js && yarn test:blueprints:ts",
     "test:node": "qunit tests/node/**/*-test.js",
     "test:browserstack": "node bin/run-browserstack-tests.js",
-    "type-check:packages": "tsc --noEmit",
-    "type-check:stable": "tsc --noEmit --project type-tests/stable",
-    "type-check:preview": "tsc --noEmit --project type-tests/preview",
+    "type-check:internals": "tsc --noEmit",
+    "type-check:types": "tsc --noEmit --project type-tests",
     "type-check": "npm-run-all type-check:*"
   },
   "dependencies": {