Add (?aT) for updating PCRE2_EXTRA_ASCII_DIGIT in pattern

[carenas]

As a follow up for #223 and affecting [:digit:] and [:xdigit:]

[PhilipHazel]

The documentation says that PCRE2_EXTRA_ASCII_POSIX affects all POSIX classes, which implies that it should not be necessary also to set PCRE2_EXTRA_ASCII_DIGIT when you set PCRE2_EXTRA_ASCII_POSIX. I agree, though, that (?a should be able to set/reset PCRE2_EXTRA_ASCII_DIGIT independently. I guess it will have to be (?aT) as (?aD) is already taken for \d.

[carenas]

I guess it will have to be (?aT)

if tried with (?aG) originally but it would seem more logical to reuse the one that applies to all POSIX classes instead as shown by the proposed changes, since that behaviour had to be modified anyway.

FWIW we already discussed this before, but what I didn't realize then is that users of PCRE2_EXTRA_ASCII_DIGIT that are not using PCRE2_EXTRA_ASCII_POSIX might want to disable its effect somehow and before this change that wasn't possible.

[PhilipHazel]

I am confused. Setting (?aP) affects all POSIX classes, including digit and xdigit. When PCRE2_EXTRA_ASCII_POSIX (aka (?aP)) is set, PCRE2_EXTRA_ASCII_DIGIT has no effect. The use case for ...DIGIT is when you just want to restrain digits but let other classes be extended. What is lacking is a (?a setting that just sets/unsets ...DIGIT.