gh-115119: Switch Windows build to mpdecimal external (GH-115182)

This includes adding what should be a relatively temporary
`Modules/_decimal/windows/mpdecimal.h` shim to choose between `mpdecimal32vc.h`
or `mpdecimal64vc.h` based on which of `CONFIG_64` or `CONFIG_32` is defined.

Misc/NEWS.d/next/Windows/2024-02-08-14-48-15.gh-issue-115119.qMt32O.rst

@@ -0,0 +1,3 @@
+Switched from vendored ``libmpdecimal`` code to a separately-hosted external
+package in the ``cpython-source-deps`` repository when building the
+``_decimal`` module.

Misc/externals.spdx.json

@@ -43,6 +43,28 @@
       "primaryPackagePurpose": "SOURCE",
       "versionInfo": "3.4.4"
     },
+    {
+      "SPDXID": "SPDXRef-PACKAGE-mpdecimal",
+      "checksums": [
+        {
+          "algorithm": "SHA256",
+          "checksumValue": "93118043651ffa33dcaaab445bae4f8929fca25d2d749079b78e97f220c3d8b1"
+        }
+      ],
+      "downloadLocation": "https://github.com/python/cpython-source-deps/archive/refs/tags/mpdecimal-2.5.1.tar.gz",
+      "externalRefs": [
+        {
+          "referenceCategory": "SECURITY",
+          "referenceLocator": "cpe:2.3:a:bytereef:mpdecimal:2.5.1:*:*:*:*:*:*:*",
+          "referenceType": "cpe23Type"
+        }
+      ],
+      "licenseConcluded": "NOASSERTION",
+      "name": "mpdecimal",
+      "originator": "Organization: bytereef.org",
+      "primaryPackagePurpose": "SOURCE",
+      "versionInfo": "2.5.1"
+    },
     {
       "SPDXID": "SPDXRef-PACKAGE-openssl",
       "checksums": [