src: support UTF-8 in compiled-in JS source files

Detect it when source files in lib/ are not ASCII.  Decode them as UTF-8
and store them as UTF-16 in the binary so they can be used as external
string resources without non-ASCII characters getting mangled.

Fixes: https://github.com/nodejs/node/issues/10673
PR-URL: https://github.com/nodejs/node/pull/11129


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>