ci: add curl-for-win builds: Linux MUSL, macOS, Windows

Linux MUSL (llvm/clang), macOS Apple clang, Windows (llvm/clang).

Configured with HTTP/2 and HTTP/3 and other dependencies (the default
curl-for-win) for a comprehensive build test.

```
curl 8.8.0-DEV (x86_64-unknown-linux-musl) libcurl/8.8.0-DEV LibreSSL/3.9.1 zlib/1.3.1 brotli/1.1.0 zstd/1.5.6 libpsl/0.21.5 libssh2/1.11.0 nghttp2/1.61.0 ngtcp2/1.4.0 nghttp3/1.2.0
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli HSTS HTTP2 HTTP3 HTTPS-proxy IPv6 Largefile libz NTLM PSL SSL threadsafe UnixSockets zstd

curl 8.8.0-DEV (x86_64-apple-darwin) libcurl/8.8.0-DEV LibreSSL/3.9.1 zlib/1.3.1 brotli/1.1.0 zstd/1.5.6 libpsl/0.21.5 libssh2/1.11.0 nghttp2/1.61.0 ngtcp2/1.4.0 nghttp3/1.2.0
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli HSTS HTTP2 HTTP3 HTTPS-proxy IPv6 Largefile libz NTLM PSL SSL threadsafe UnixSockets zstd

curl 8.8.0-DEV (x86_64-w64-mingw32) libcurl/8.8.0-DEV LibreSSL/3.9.1 zlib/1.3.1 brotli/1.1.0 zstd/1.5.6 WinIDN libpsl/0.21.5 libssh2/1.11.0 nghttp2/1.61.0 ngtcp2/1.4.0 nghttp3/1.2.0
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli HSTS HTTP2 HTTP3 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM PSL SPNEGO SSL SSPI threadsafe UnixSockets zstd
```

Limited to x64, because for build testing the additional CPUs don't add
much value compared to the extra build time. They can be enabled easily
if deemed useful.

To the extent of curl-for-win configuration options, it's trivial to add
further build combinations.

Closes #13335

.github/workflows/curl-for-win.yml

@@ -0,0 +1,91 @@
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+---
+name: curl-for-win
+
+on:
+  push:
+    branches:
+      - master
+      - '*/ci'
+  pull_request:
+    branches:
+      - master
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+permissions: {}
+
+env:
+  CW_NOGET: 'curl trurl'
+  CW_MAP: '0'
+  CW_JOBS: '3'
+  CW_NOPKG: '1'
+  DOCKER_CONTENT_TRUST: '1'
+
+jobs:
+  linux-musl-llvm:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          path: 'curl'
+          fetch-depth: 8
+      - name: 'build'
+        run: |
+          git clone --depth 1 https://github.com/curl/curl-for-win
+          mv curl-for-win/* .
+          export CW_CONFIG='-main-werror-linux-musl-x64'
+          export CW_REVISION='${{ github.sha }}'
+          . ./_versions.sh
+          docker trust inspect --pretty "${DOCKER_IMAGE}"
+          time docker pull "${DOCKER_IMAGE}"
+          docker images --digests
+          time docker run --volume "$(pwd):$(pwd)" --workdir "$(pwd)" \
+            --env-file <(env | grep -a -E \
+              '^(CW_|GITHUB_)') \
+            "${DOCKER_IMAGE}" \
+            sh -c ./_ci-linux-debian.sh
+
+  mac-clang:
+    runs-on: macos-latest
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          path: 'curl'
+          fetch-depth: 8
+      - name: 'build'
+        run: |
+          git clone --depth 1 https://github.com/curl/curl-for-win
+          mv curl-for-win/* .
+          export CW_CONFIG='-main-werror-mac-x64'
+          export CW_REVISION='${{ github.sha }}'
+          sh -c ./_ci-mac-homebrew.sh
+
+  win-llvm:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          path: 'curl'
+          fetch-depth: 8
+      - name: 'build'
+        run: |
+          git clone --depth 1 https://github.com/curl/curl-for-win
+          mv curl-for-win/* .
+          export CW_CONFIG='-main-werror-win-x64'
+          export CW_REVISION='${{ github.sha }}'
+          . ./_versions.sh
+          docker trust inspect --pretty "${DOCKER_IMAGE}"
+          time docker pull "${DOCKER_IMAGE}"
+          docker images --digests
+          time docker run --volume "$(pwd):$(pwd)" --workdir "$(pwd)" \
+            --env-file <(env | grep -a -E \
+              '^(CW_|GITHUB_)') \
+            "${DOCKER_IMAGE}" \
+            sh -c ./_ci-linux-debian.sh