Don't clobber std::string_view equality with char *

Make the wxString(std::string_view) constructor explicit.

Otherwise, when comparing a std::string_view with a const char *, the
cast to wxString will be considered as a candidate for the comparison,
ultimately causing an "ambiguous overload for 'operator=='" error.

For example, this sample only builds if the constructor is explicit:

  #include <wx/string.h>
  #include <string_view>

  int main() {
    std::string_view view = "abc";
    const char *str = "abc";
    return view == str;
  }

However, making the constructor explicit will break assignment:

    std::string_view view = "abc";
    wxString s;
    s = view; // Error: no match for "operator="

That we can fix by implementing operator=(std::string_view)

That, however, introduces another ambiguity:

    std::string str = "abc";
    wxString s;
    s = str; // Ambiguous between s = wxString(str)
                              and s = std::string_view(str)

That we can fix by implementing operator=(std::string)

Finally, note that some rather obscure ambiguities remain, such as:

    wxString s;
    s = {"abc", 2}; // Ambiguous between s = wxString("abc", 2)
                                     and s = std::string_view("abc", 2)

Avoiding them is not simple (https://cplusplus.github.io/LWG/issue2946)
and doesn't add much value.

Closes #23834.

docs/changes.txt

@@ -108,6 +108,11 @@ Changes in behaviour which may result in build errors
   compatible with the previous wxWidgets versions, but now compare values, and
   not pointers, in their Index() member function.
 
+- Due to the possibility to construct wxString from std::string_view some
+  previously valid code, such as "wxstr = {"Hello", 2}", is now ambiguous.
+  Please use explicit class name, e.g. "wxstr = wxString{"Hello", 2}" to
+  preserve the previous behaviour.
+
 - Generic wxSearchCtrl doesn't provide methods that make sense only for
   multiline text controls any longer, for consistency with the other ports.