build(windows): restore tee.exe on Windows #36363

Problem:
Neovim no longer ships with a tee binary on Windows, which breaks
functionality for the :grep and :make commands.

    nvim --clean
    :grep foo or :make
    "tee is not recognized as an internal or external command"

Solution:
Include a simple, no-dependency tee.c source file in the src/ directory.
Update CMakeLists.txt to build a tee executable alongside neovim during
the build process, and ensure the tee.exe program appears alongside the
neovim executable in the bin/ directory so that it is accessible for
:grep and :make.

tee.c was obtained from the vim codebase:
https://github.com/vim/vim/blob/master/src/tee/tee.c

And we modified it to fix performance issues.

Testing:

    nvim --clean
    :grep foo or :make, after setting a file to the makeprg option.
    Verify that :grep results and error output from a compiler appear in the message pane.

ref https://github.com/neovim/neovim/issues/32431
fix https://github.com/neovim/neovim/issues/32504

Other tee options:
- [tee-win32](https://github.com/dEajL3kA/tee-win32): MIT. However,
  I couldn't get it to build on my machine even after updating its
  makefile to call my install of MSVC. It's also super optimized and
  uses some processor intrinsics for multithreading.
- [gnu coreutils tee](https://gnuwin32.sourceforge.net/packages/coreutils.htm):
  (Windows coreutils contains a tee.c. Last updated 2005. Did not build
  immediately on my machine; we'd have to determine which definitions
  from elsewhere in coreutils tee.c needs and incorporate them somehow.
- [WinTee](https://github.com/mpderbec/WinTee): Has no license. Last
  updated 11 years ago. Relies on Visual Studio to build.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

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