In-place JS environment

Summary:
This change restructures the package to try to remove the JS build step from the inner loop. Instead, we have a single `src` directory that we babel transform when using, then apply the same transform inline during prepublish.

At the end, we will be publishing a source directory with Babel transformed TS, JS, and TS declarations.

We do a little spring cleaning when doing this. Fixing up some of the folder/file conventions, and removing the non-export-map fallbacks.

We cannot remove the need for a native build.

Reviewed By: yungsters

Differential Revision: D45682153

fbshipit-source-id: ea2dd75c2dd6e3529b1ef6cf6ac6a64a270049a4

javascript/tsconfig.json

@@ -3,13 +3,13 @@
     "target": "es2018",
     "module": "commonjs",
     "strict": true,
+    "declaration": true,
     "esModuleInterop": true,
     "skipLibCheck": true,
     "forceConsistentCasingInFileNames": true,
-    "noEmit": true,
     "baseUrl": ".",
     "paths": {
-      "yoga-layout": ["src_js"]
+      "yoga-layout": ["src"]
     }
   },
   "ts-node": {