Doc: document emcascript features we use (#1763)

EMCASCRIPT.md

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+# ECMAScript and Node requirements
+
+The goal of the document is to list features we rely on for the runtime and the generated code.
+Features are grouped by ECMAScript version.
+
+## ECMAScript 2015 - ES6
+### rest parameters
+- added in Nodejs 6
+### Object.assign
+- added In Nodejs 4
+### ArrowFunction
+- for wasm_of_ocaml
+### TypedArray
+- to implement bigarray
+
+## ECMAScript 2016
+### Async/Await
+- for wasm_of_ocaml
+
+## ECMAScript 2020
+### globalThis
+- added in Nodejs 12
+- polyfill-ed in the repo
+
+## ECMAScript 2021
+### WeakRef to implement weak and ephemeron
+- added in Nodejs 14.6
+- optional
+
+## Node
+### Namespaced require `require("node:fs")`
+- added in Nodejs 16