Add better package declaration specific error message (#878)

src/parser.cpp

@@ -5204,6 +5204,11 @@ bool parse_file(Parser *p, AstFile *f) {
 
 	CommentGroup *docs = f->lead_comment;
 
+	if (f->curr_token.kind != Token_package) {
+		syntax_error(f->curr_token, "Expected a package declaration at the beginning of the file");
+		return false;
+	}
+
 	f->package_token = expect_token(f, Token_package);
 	if (f->package_token.kind != Token_package) {
 		return false;