Use std::string in rb_phone_number_area_code

[RemcodM]

When installing mini_phone on macOS 15 (Sequoia) using libphonenumber version 8.13.46, I get the following errors:

checking for -lphonenumber... yes
checking for -lprotobuf... yes
checking for whether -O3 is accepted as CPPFLAGS... yes
creating Makefile

current directory: /Users/user/.rvm/gems/ruby-3.3.5/gems/mini_phone-1.2.0/ext/mini_phone
make DESTDIR\= sitearchdir\=./.gem.20240930-25052-gju52q sitelibdir\=./.gem.20240930-25052-gju52q clean

current directory: /Users/user/.rvm/gems/ruby-3.3.5/gems/mini_phone-1.2.0/ext/mini_phone
make DESTDIR\= sitearchdir\=./.gem.20240930-25052-gju52q sitelibdir\=./.gem.20240930-25052-gju52q
compiling mini_phone.cc
mini_phone.cc:420:3: error: unknown type name 'string'; did you mean 'std::string'?
  420 |   string national_significant_number;
      |   ^~~~~~
      |   std::string
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:48:7: note: 'std::string' declared here
   48 | using string = basic_string<char>;
      |       ^
mini_phone.cc:422:3: error: unknown type name 'string'; did you mean 'std::string'?
  422 |   string area_code;
      |   ^~~~~~
      |   std::string
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:48:7: note: 'std::string' declared here
   48 | using string = basic_string<char>;
      |       ^
mini_phone.cc:423:3: error: unknown type name 'string'; did you mean 'std::string'?
  423 |   string subscriber_number;
      |   ^~~~~~
      |   std::string
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:48:7: note: 'std::string' declared here
   48 | using string = basic_string<char>;
      |       ^
mini_phone.cc:428:78: error: use of undeclared identifier 'string'; did you mean 'std::string'?
  428 |     subscriber_number = national_significant_number.substr(area_code_length, string::npos);
      |                                                                              ^~~~~~
      |                                                                              std::string
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:48:7: note: 'std::string' declared here
   48 | using string = basic_string<char>;
      |       ^
4 errors generated.
make: *** [mini_phone.o] Error 1

It seems that rb_phone_number_area_code function specifically does not use std::string everywhere and just uses string. I suspect that a macOS update somewhere does not allow this anymore, failing new installs.

Replacing string with std::string seems to solve the compiler issues.

[RemcodM]

Any updates?