std: User a smaller stdin buffer on windows

Apparently windows doesn't like reading from stdin with a large buffer size, and
it also apparently is ok with a smaller buffer size. This changes the reader
returned by stdin() to return an 8k buffered reader for stdin rather than a 64k
buffered reader.

Apparently libuv has run into this before, taking a peek at their code, with a
specific comment in their console code saying that "ReadConsole can't handle big
buffers", which I presume is related to invoking ReadFile as if it were a file
descriptor.

Closes #13304