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/Size.h

@@ -0,0 +1,17 @@
+/*
+ * 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.
+ */
+
+#pragma once
+
+struct Size {
+  double width;
+  double height;
+
+  Size(void) : width(0.0), height(0.0) {}
+
+  Size(double width, double height) : width(width), height(height) {}
+};