The formatting in this project has become something of a mess,
because it changed over time. Add a CS fixer config and reformat
to the desired style, which is PSR-12, but with sane brace placement.
Add ErrorHandler interface, as well as ErrorHandler\Throwing
and ErrorHandler\Collecting. The error handler is passed to
Parser::parse(). This supersedes the throwOnError option.
NameResolver now accepts an ErrorHandler in the ctor.
