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.

tests/strings/stdstrings.cpp

@@ -690,5 +690,14 @@ TEST_CASE("StdString::View", "[stdstring]")
     std::string_view strViewInvalidUTF(strInvalidUTF);
 
     CHECK( "" == wxString::FromUTF8(strViewInvalidUTF) );
+
+    /* Ensure we don't clobber comparisons on base types */
+    std::string_view view = "abc";
+    const char *str = "abc";
+    CHECK( view == str );
+
+    std::wstring_view wview = L"abc";
+    const wchar_t *wstr = L"abc";
+    CHECK( wview == wstr );
 }
 #endif // wxHAS_STD_STRING_VIEW