Lexer fix for DoubleLiteral
Lexer fix for DoubleLiteral

Tokenising the input `.6f 0.6f 1.6f` results in these tokens:

```
RealLiteral               `.6f`
IntegerLiteral            `0`
RealLiteral               `.6f`
RealLiteral               `1.6f`
```

I.e. `0.6f` is not properly recognized as a `DoubleLiteral`.
antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4