Implements https://tc39.github.io/proposal-hashbang/, which simply
ignores the first line of a source file if it begins with '#!'
(U+0023 U+0021).

The test cases are influenced by
https://github.com/tc39/test262/pull/1983, which have not been pulled
into test262 local-tests due to issues with parseTestRecord.

BUG=v8:8523
R=gsathya@chromium.org, adamk@chromium.org, littledan@chromium.org

Change-Id: I4ae40222298de768a170c7a1d45fec118ed5713c
Reviewed-on: https://chromium-review.googlesource.com/c/1409527
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58838}