Function optional

optional<D, E>(parser: TokenParser<D, E>): TokenParser<D, E>
It defines a TokenParser that will match parser if it matches. If parser does not match, it will not consume any input and stay in place.

It works like the question mark ( ? ) operator in regular expressions.

The same behavior is implemented for SymbolParser in the following way:
```
 const optional_a: SymbolParser<null, null> = alternatives([
     recursive(str('a')),
     empty,
 ]);
```
Returns
A new TokenParser that will optionally match parser.
Type Parameters
- D extends Identifiable
- E extends Identifiable
Parameters
- parser: TokenParser<D, E>
  
  The parser to be optionally matched.
Returns TokenParser<D, E>
- Defined in index.ts:1015