Revive JavaScript Bindings (#1177)

Summary:
Yoga's JavaScript bindings do not work past Node 10, or on recent versions of Ubuntu even using it. This is due to a reliance on `nbind`, a library which is no longer maintained. `nbind` itself abstracts over `embind` running Emscripten to generate an asm.js build, along with building Node native modules. In the meantime, [yoga-layout-prebuilt](https://www.npmjs.com/package/yoga-layout-prebuilt) has been used by the community instead of the official package.

https://github.com/facebook/yoga/pull/1177 was contributed as a conversion of bindings created using `nbind` to instead use `embind` directly.

I continued building on this to add more:
1. WebAssembly support (required to be async in browsers)
2. CMake + Ninja Build for the 4 flavors
3. TypeScript typings (partially generated)
4. yarn scripts to build (working on macOS, Ubuntu, Windows)
5. A README with some usage and contribution instructions
6. Updated tests to work with Jest, and updated general infra
7. ESLint and clang-format scripts
8. More GitHub actions (and now testing Windows)
9. Probably more I kinda got carried away here lol

The plan is to eventually publish this to NPM, but there is a little bit of work after this before that happens.

Pull Request resolved: https://github.com/facebook/yoga/pull/1177

Test Plan: The bindings pass Jest tests (both manual and generated). GitHub actions added for the different yarn scripts. Did some manual checks on using the library as TS.

Reviewed By: christophpurrer

Differential Revision: D42207782

Pulled By: NickGerleman

fbshipit-source-id: 1dc5ce440f1c2b9705a005bbdcc86f952785d94e

javascript/README.md

@@ -0,0 +1,76 @@
+# yoga-layout
+
+This package provides prebuilt JavaScript bindings for the Yoga layout engine. Both WebAssembly and asm.js variants are packaged, with the optimal loaded based on platform.
+
+## Usage
+
+The default entrypoint provides an asynchronous loader function to return a Yoga instance.
+
+```ts
+import { loadYoga, ALIGN_CENTER } from "yoga-layout";
+
+const Yoga = await loadYoga();
+
+const node = Yoga.Node.create();
+node.setAlignContent(ALIGN_CENTER);
+```
+
+An alternative synchronous API is provided for compatibility, but requires using asm.js in browsers instead of WebAssembly, leading to worse performance and larger assets.
+
+```ts
+import Yoga, { ALIGN_CENTER } from "yoga-layout/sync";
+
+const node = Yoga.Node.create();
+node.setAlignContent(ALIGN_CENTER);
+```
+
+Objects created by `Yoga.<>.create()` are not automatically garbage collected and should be freed once they are no longer in use.
+
+```ts
+// Free a config
+config.free();
+
+// Free a tree of Nodes
+node.freeRecursive();
+
+// Free a single Node
+node.free();
+```
+
+## Selecting WebAssembly or asm.js
+
+For better performance and smaller packages, WebAssembly is preferred to asm.js where available. `yoga-layout` tries to provide the right default using [export maps](https://webpack.js.org/guides/package-exports/#conditional-syntax) so that platforms which can take advantage of WebAssembly use it by default.
+
+A specific entrypoint may be specified on platforms which do not understand export conditions.
+
+```ts
+import { loadYoga } from "yoga-layout/dist/entrypoint/wasm-async";
+```
+
+
+## Contributing
+
+### Requirements
+
+1. Emscripten SDK
+1. CMake >= 3.13
+1. (Optional) ninja, for faster builds
+
+### Building
+
+```bash
+git clone https://github.com/facebook/yoga.git
+cd yoga/javascript
+yarn install
+yarn build
+```
+
+### Testing
+
+```bash
+# Build and test all entrypoints
+yarn test
+
+# Build and test a specific entrypoint
+yarn test:asmjs-sync
+```