Add wxInt64 support to wxText{Input,Output}Stream.

Add explicit Read64[S]() and Write64() methods.

Closes #14685.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76171 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775