do not allow _ at start of identifier

parser/src/parser/context.rs

@@ -302,6 +302,10 @@ impl ParserContext {
                     span,
                 } = token
                 {
+                    if name.starts_with('_') {
+                        return Err(SyntaxError::invalid_ident_name(&name, &name[1..name.len()], &span));
+                    }
+
                     Ok(Identifier { name, span })
                 } else {
                     unimplemented!()