Provide a reliable way to find the entrypoint path or URL

### What is the problem this feature will solve? This solves the same problem as `require.main.filename` did in a CommonJS-only world: finding the entrypoint script for the Node.js process. This is most commonly used to determine the whether the current file is the entrypoint, but is also useful for various other sorts of introspection about the running Node.js environment. My specific use-case is finding a sensible default base URL to use to replicate Node.js's package resolution algorithm. ### What is the feature you are proposing to solve the problem? I suggest something like `process.cliEntrypoint` or `process.cliMainUrl`. I think it's important that any solution be usable _regardless_ of whether the current module is CJS or ESM. ### What alternatives have you considered? `require.main.filename` provides this information but it doesn't work outside of CJS files, nor does it work if the entrypoint itself is ESM. `process.argv[1]` _sometimes_ provides this information, but it has a number of problems: * If the Node.js CLI is run without a file extension, like `node myapp` instead of `node myapp.js`, `process.argv[1]` won't have an extension either. * If the entrypoint is a symlink, `process.argv[1]` will point to the symlink instead of the physical JS file. This differs from `__filename`, which means it also differs from the logic used by `require()`. Note that `npm exec` _always_ creates a symlink when running executables. * `process.argv` is mutable, so it's possible for unrelated code to modify it in such a way that it's no longer usable to find the entrypoint.