d8 treats files with the .mjs extension as modules instead of
classic scripts. Thus, the `// MODULE` pragma and its corresponding
logic in test runners can be removed in favor of explicitly adding
the extension.

Bug: v8:7950, v8:9395, v8:9406
Also-By: tmrts@chromium.org
Change-Id: Ic74328dc5c5f176bb4bdf6d74bdd4d3966279ba5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1675958
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62421}

This way, we can also check the return code of d8. We currently have a
bug (6981) which makes failing tests not being detected, even though
the failure message is (sometimes) being printed.
After this refactoring, we can write tests for our mjsunit test
functions.

R=machenbach@chromium.org

Bug: v8:6981
Change-Id: I0aa0abcb0f9a4f622a1e1d1a4d826da1e6eb4f07
Reviewed-on: https://chromium-review.googlesource.com/737991Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48951}

The new logic ensures that the error messages are the same in the
"import { <reserved word> }" and "import { foo as <reserved ord> }"
cases.

Also prepares ParseImportNames for returning both the import and local
names to ParseImportClause.

BUG=v8:1569
LOG=n

Review URL: https://codereview.chromium.org/952863006

Cr-Commit-Position: refs/heads/master@{#26863}