Proposal: Have the ESM loader handle all entry points

In current Node, the ESM loader handles the application entry point when it is an ES module _or_ if any of the following flags are passed: `--import`, `--experimental-default-type`, `--experimental-detect-module`, `--experimental-loader`. This means that the ESM loader handles CommonJS entry points when certain flags are passed. (The ESM loader supports ES modules, CommonJS modules, WebAssembly modules and JSON files.) A function [`shouldUseESMLoader` in `run_main.js`](https://github.com/nodejs/node/blob/0fb512344fe97b508adf1332c3e82d33937c4669/lib/internal/modules/run_main.js#L52-L87) determines whether the ESM loader should handle the main entry point. If we were to remove this function and simply _always_ have the ESM loader handle all entry points, there would be a few gains: - There would be only one code path for starting the application entry point, rather than two, simplifying the codebase and making it more maintainable. - The simpler the codebase can become, the more likely it will be that we can find performance improvements, such as by moving parts of the flow into C++. It would also mean the end of monkey-patching the CommonJS loader, which has never been officially supported and I think is ready to be officially retired now that the [module customization hooks](https://nodejs.org/api/module.html#customization-hooks) exist and provide fully equivalent functionality, including for customizing `require` calls. They are Release Candidate and I expect them to become stable in the next few months. This potential change of having the ESM loader handle all entries would happen after that, at the next major version of Node. It _shouldn’t_ be a semver-major change, but monkey-patching is common enough that I’d expect some breakage and therefore we should treat it as semver-major. If you take the [`run_main.js` `if (useESMLoader) {` line](https://github.com/nodejs/node/blob/0fb512344fe97b508adf1332c3e82d33937c4669/lib/internal/modules/run_main.js#L130) and change…