Add regex ASCII flag support for matching builtin character classes

[davidwendt]

Description

Adds ASCII flag to the libcudf regex_flags for support with builtin character classes: \w, \W, \s, \S, \d, \D. Somewhat equivalent to https://docs.python.org/3/library/re.html#re.ASCII But strictly the flag modifies matching for these classes as follows:

• \w = [a-zA-Z_] (alphabetic characters plus underline)
• \W = [^\w] (basically not \w)
• \s = [\t- ] (tab through space in the ASCII table)
• \S = [^\s] (basically not \s)
• \d = [0-9] (digit characters)
• \D = [^\d] (basically not \d)

Additional gtests are included for this flag with these classes. This will be exposed through Python/Cython in a follow up PR.

Closes #10894

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

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[davidwendt]

Does nobody use the regex engine from the C++ side? I am just surprised we don't have any API docs that explain the regex keywords we support.

Spark uses the C++ regex code. We have some good libcudf documentation here: https://docs.rapids.ai/api/libcudf/stable/md_regex.html

Looks like I should update it for this new flag.

[davidwendt]

@gpucibot merge