Change set of permissible Unicode letters in identifiers

GHC deviates from the report in the set of Unicode character classes that are allowd in the syntax of identifiers. This is documented here: https://downloads.haskell.org/ghc/latest/docs/users_guide/bugs.html#lexical-syntax

> GHC is more lenient in which characters are allowed in the identifiers. Unicode Other Letters are considered to be small letters, therefore variable identifiers can begin with them. Digit class contains all Unicode numbers instead of just Decimal Numbers. Modifier Letters and Non-Spacing Marks can appear in the tail of the identifiers.:

```
uniSmall → any Unicode Lowercase Letter or Other Letter
uniDigit → any Unicode Decimal Number, Letter Number or Other Number

uniIdchar → any Unicode Modifier Letter or Non-Spacing Mark
idchar → small | large | digit | uniIdchar | '

varid → small {idchar} ⟨reservedid⟩
conid → large {idchar}
```

Start with the linked lexical-syntax documentation and locate the report's identifier grammar. Compare its Unicode character classes with the productions listed in the issue, then update the report so the permissible classes and accompanying explanation are consistent. Done means the revised syntax clearly reflects the intended Unicode behavior.

Written by the indexing model from the issue text.