feat(frontends/lean/scanner): disallow hierarchical names such as 'x.1'

src/frontends/lean/scanner.cpp

@@ -354,7 +354,7 @@ auto scanner::read_key_cmd_id() -> token_kind {
             } else if (cs[i] == '.') {
                 next_utf(cs);
                 num_utfs++;
-                if (!is_id_rest(cs, i+1))
+                if (!is_id_first(cs, i+1))
                     break;
             } else {
                 break;

tests/lean/bad_id.lean

@@ -0,0 +1,5 @@
+import data.num
+
+definition x.y := 10
+
+definition x.1 := 10

tests/lean/bad_id.lean.expected.out

@@ -0,0 +1 @@
+bad_id.lean:5:12: error: invalid binder, '(', '{', '[', '{{', '⦃' or identifier expected