Fixes according to chapter Lexical of the Language Reference

- line comment is not recognized at end of file
- `WysiwygString` with non-empty content is not recognized
- escape sequence `\?` is not recognized
- escape sequence with single octal digit is not recognized
- named character entities are not recognized
- `DecimalFloat` is not recognized when it starts with `.` or does not contain `.`
- `SpecialTokenSequence` is very special:
   the compiler requires `#line 42` instead of `#line42` and allows `#line 42 (foo)`

By the way: why don't you use a grammar rule `stringLiterals` according to the chapter Expressions?
Your `StringFragment`s must also allow for comments or even `SpecialTokenSequence`s in between.