Optional Feature: backtracking is sub-calls

```
  IsMatching('Recursive', '^(?''A''(?:b[^b]+(?&A)?x+))', 'baaabaaaaaxx', [1,12,  1,12]);
```

https://regex101.com/r/g3pDSW/1

```
^(?'A'
  (?:
    b[^b]+
    (?&A)?  ## This sub-call will match ALL the "x", but when the outer match fails, it backtracks, and matches only ONE "x"
     x+
  )
)
```

TRegExpr fails to match, because it does not backtrack the sub-call.
