analysis: remove 'else', replace by dedicated Block

It isn't possible to make Blocks at root scope. the parser does not
allow such constructions, however, it is still a Node, and it should be
handled accordingly.

src/analysis.zig

@@ -550,9 +550,9 @@ pub const Analyzer = struct {
                 }
             },
 
-            else => {
-                std.debug.warn("TODO type analysis of {}\n", node.*);
-                return CompileError.TypeError;
+            .Block => {
+                self.doError("Block can't be found at root");
+                return CompileError.Invalid;
             },
         }
     }