Entry point without TLA for js package (#1638)

Summary:
Follow up on https://github.com/facebook/yoga/issues/1637

TLDR: tooling for TLA is not there yet; An additional entry point without top-level-await is appropriate

- adds ./load entry to js package
- uses .js file extensions to prevent requiring [allowImportingTsExtensions](https://www.typescriptlang.org/tsconfig#allowImportingTsExtensions)

Pull Request resolved: https://github.com/facebook/yoga/pull/1638

Reviewed By: joevilches

Differential Revision: D55614636

Pulled By: NickGerleman

fbshipit-source-id: 126a94aa68d22d32b938282cfa1a5059bb9df337

javascript/README.md

@@ -29,3 +29,12 @@ node.free();
 ## Requirements
 
 `yoga-layout` requires a toolchain that supports ES Modules and top-level await.
+
+If top-level-await is not supported, use the `yoga-layout/load` entry point instead. This requires to load yoga manually:
+
+```ts
+import {loadYoga, Align} from 'yoga-layout/load';
+
+const node = (await loadYoga).Node.create();
+node.setAlignContent(Align.Center);
+```