This patch makes `d8` recognize files with the `.mjs` extension as
modules instead of classic scripts. This change can be tested by saving
the following JavaScript program as both `module.mjs` and as
`script.js`:

    console.log(this === undefined ? 'strict' : 'sloppy');

Then, run these files in `d8` without passing the `--module` flag:

    $ d8 module.mjs
    strict

    $ d8 script.js
    sloppy

The use of `.mjs` matches not just Google’s recommendation [1] but also
the current modules implementation in Node.js [2].

[1] https://developers.google.com/web/fundamentals/primers/modules
[2] https://nodejs.org/api/esm.html

Bug: v8:7950
Change-Id: I8f39420dc24a5eedd7e88d3b1aa48207ebfeff6e
Reviewed-on: https://chromium-review.googlesource.com/1140314
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54502}