Improve error reporting for illegal variable names

[mmarx]

Improve the reporting of illegal variable names, e.g., in fstrings (previously, this would report “expected )” at the opening "):

[01] Error: expected `letter or digit`
   ╭─[ /dev/shm/foo.rls:2:15 ]
   │
 2 │ foo(f"test: {?_test}") :- bar(_), ?_test = 42.
   │               │
   │               ╰─ expected `letter or digit`
───╯

[aannleax]

Looks good :)