doc: add initial doc on how to update cjs-module-lexer

doc/contributing/maintaining-cjs-module-lexer.md

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+# Maintaining cjs-module-lexer
+
+The [cjs-module-lexer](https://github.com/nodejs/node/tree/master/deps/cjs-module-lexer)
+dependency is used within the Node.js ESM implementation to detect the
+named exports of a CommonJS module.
+
+It is used within
+[lib/internal/modules/esm/translators](https://github.com/nodejs/node/blob/master/lib/internal/modules/esm/translators.js)
+in which both `internal/deps/cjs-module-lexer/lexer.js` and
+`internal/deps/cjs-module-lexer/dist/lexer.js` are required and used.
+
+`internal/deps/cjs-module-lexer/lexer.js`
+is a regular JavaScript implementation that is
+used when WebAssembly is not available on a platform.
+`internal/deps/cjs-module-lexer/dist/lexer.js` is a faster
+implementation using WebAssembly which is generated from a
+C based implementation. These two paths
+resolve to the files in `deps/cjs-module-lexer` due to their
+inclusion in the `deps_files` entry in
+[node.gyp](https://github.com/nodejs/node/blob/master/node.gyp).
+
+The two different versions of lexer.js are maintained in the
+[nodejs/cjs-module-lexer](https://github.com/nodejs/cjs-module-lexer) project.
+
+In order to update the Node.js dependencies to use to a newer verion
+of cjs-module-lexer, complete the following steps:
+
+* Clone [nodejs/cjs-module-lexer](https://github.com/nodejs/cjs-module-lexer)
+  and check out the version that you want Node.js to use.
+* Follow the WASM build steps outlined in
+  [wasm-build-steps](https://github.com/nodejs/cjs-module-lexer#wasm-build-steps).
+  This will generate the WASM based dist/lexer.js file.
+* Preserving the same directory structure, copy the following files over
+  to deps/cjs-module-lexer directory where you have checked out Node.js
+
+```text
+├── CHANGELOG.md
+├── dist
+│   ├── lexer.js
+│   └── lexer.mjs
+├── lexer.js
+├── LICENSE
+├── package.json
+└── README.md
+```
+
+* Update the link to the cjs-module-lexer in the list at the end of
+  [doc/api/esm.md](https://github.com/nodejs/node/blob/master/doc/api/esm.md)
+  to point to the updated version.
+
+* Create a PR, adding the files in the deps/cjs-module-lexer that
+  were modified.
+
+If updates are needed to cjs-module-lexer for Node.js, first PR
+those updates into
+[nodejs/cjs-module-lexer](https://github.com/nodejs/cjs-module-lexer),
+request a release and then pull in the updated version once available.