Disallow Unicode in numeric literals.

The Haskell report allows Unicode digits in numeric literals, but as documented in https://downloads.haskell.org/ghc/latest/docs/users_guide/bugs.html#lexical-syntax GHC disallows this. GHC only allows ASCII digits:
```
ascDigit → 0 | 1 | … | 9
decimal → ascDigit {ascDigit}
```

Read the linked GHC lexical-syntax note and locate the numeric-literal grammar in the revised report. Compare its treatment of digits with the ASCII-only rules quoted in the issue; done means the report no longer permits Unicode digits in numeric literals.

Written by the indexing model from the issue text.