<locale>: std::ctype<char>::blank should return true only for space and tab

[Arzaghi]

Fixes #1121
It's a very simple and probably a temporary solution to this issue. The actual fix should be in the UCRT(I think) and it also needs some modifications in character classification flags. According to This Table, _Ctype._Table[static_cast<unsigned char>('\t')] should be 104 (0x68). But now it returns 40 (0x28) just like the other white-space control codes ('\f','\v','\n','\r'). So, I think there is no direct way to distinguish them without touching the UCRT.
In this fix, I've tried to treat blank as a special case. But some test cases failed in Regex test cases. So, I've made some modifications to the <regex> too.

[StephanTLavavej]

I believe that there will be a merge conflict with #1307 which is reducing the scope of the extremely short _SP family of macros.

[Arzaghi]

I believe that there will be a merge conflict with #1307 which is reducing the scope of the extremely short _SP family of macros.

That's right. Furthermore, I've just found another problem with the last commit. I'm working on it. I'm going to convert it as a draft for now.

Update:
It's impossible to fix this issue without touching UCRT or breaking the ABI by modifying char flags in xlocale 😕

[StephanTLavavej]

Update: It's impossible to fix this issue without touching UCRT or breaking the ABI by modifying char flags in xlocale

Ok, thanks for investigating. We're going to close this PR and mark the linked bug as vNext so we can investigate it when ABI breaks are possible.