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/package.json

@@ -11,6 +11,10 @@
   },
   "type": "module",
   "main": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./load": "./src/load.ts"
+  },
   "files": [
     "binaries/**",
     "src/**"