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/src/entrypoint/wasm-async.js

@@ -0,0 +1,11 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+const asm = require('../build/wasm-async');
+module.exports = require('./_entryAsync')(asm);