SML lexer has some strange mis-highlights when using symbol idents

```
val x = fn y => z
val $ = fn y => z
```

The first `fn` is highlighted as Keyword::Reserved, while the second is Name::Variable. Will need to review SML syntax to fix properly - I wrote this lexer myself in 2013 and it contains some very bad decisions in hindsight.

https://rouge.jneen.ca/v5.0.0/sml/dmFsIHggPSBmbiB5ID0-IHoKdmFsICQgPSBmbiB5ID0-IHo