LibJS: Explicitly instantiate Parser::parse_function_node

Due to macOS visibility rules, this function did not end up being
exported from liblagom-js.dylib, causing LagomWeb to fail to link.

Userland/Libraries/LibJS/Parser.cpp

@@ -4582,4 +4582,7 @@ Parser::ForbiddenTokens Parser::ForbiddenTokens::forbid(std::initializer_list<To
     return result;
 }
 
+template NonnullRefPtr<FunctionExpression> Parser::parse_function_node(u8, Optional<Position> const&);
+template NonnullRefPtr<FunctionDeclaration> Parser::parse_function_node(u8, Optional<Position> const&);
+
 }