Regex parser doesn't ignore vertical tabs in patter when using RegexOptions.IgnorePatternWhiteSpace

While porting PCRE2 tests suite, one of the failing tests is:

```c#
bool isMatch = Regex.IsMatch("ab", "a\vb", RegexOptions.IgnorePatternWhitespace);
Assert.True(isMatch);
```

This should probably be true and match other engines like PCRE as `\v` character should be ignored as whitespace and just have the pattern be `ab`

cc: @stephentoub 