Problem:
Currently we fetch a prebuilt xxd.exe from neovim/deps for Windows,
which is not ideal in terms of obviousness, misses updates, and is
clumsy to update.
Solution:
Similar to tee.c (#36363), vendor xxd.c from Vim and build it as part
of the Neovim build process. This makes the source obvious, enables
analyzer checks, and simplifies updates.
Fixes#36664
