Compiler does not reject declarations of $-prefixed symbols

The following code succeeds, though these symbols can't be used, for example, in defining expressions:
```form
On names;
S $s;
V $v;
I $i;
F $f;
Set $ss;
.end
```
```
FORM 5.0.0-beta.1 (Jan 23 2026, v5.0.0-beta.1-319-gd776835)  Run: Mon Jan 26 17:00:35 2026

 Symbols
   $s
 Indices
   $i
 Vectors
   $v
 Functions
   $f
 Sets
   $ss:
```