backport: build(windows): vendor xxd.c (#36755)

backport https://github.com/neovim/neovim/pull/36746

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

Co-authored-by: benarcher2691 <ben.archer2691@gmail.com>

test/functional/editor/xxd_spec.lua

@@ -0,0 +1,19 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local eq = t.eq
+local clear = n.clear
+local fn = n.fn
+local testprg = n.testprg
+
+describe('xxd', function()
+  before_each(clear)
+
+  it('works', function()
+    -- Round-trip test: encode then decode should return original
+    local input = 'hello'
+    local encoded = fn.system({ testprg('xxd') }, input)
+    local decoded = fn.system({ testprg('xxd'), '-r' }, encoded)
+    eq(input, decoded)
+  end)
+end)