This was never legal; the spec only allows '\0' in strict-mode strings or templates
when not followed by a decimal digit. Previously we were only enforcing that it
not be followed by an _octal_ digit.

This was already fixed for numeric literals, but not for escape sequences in strings.

BUG=v8:6504

Review-Url: https://codereview.chromium.org/2950633002
Cr-Commit-Position: refs/heads/master@{#46046}