Types omitted in two places where we violate them currently:
Namespace_::$stmts can be null during parsing, and Enum_::$scalarType
can be a complex type for invalid programs.
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.
