Move Vim's CI YAML files to a separate unused folder

.github/workflows/vim_unused/README.md

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+This contains Vim's CI YAML files that are not used in MacVim. These files are
+moved to an unused spot rather than deleted so that: 1) they can be used as
+reference, and 2) if upstream has made changes to them, a Git merge will
+resolve cleanly as rename detection will resolve this.

.github/workflows/vim_unused/ci.yml

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+name: GitHub CI
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+
+# Cancels all previous workflow runs for pull requests that have not completed.
+concurrency:
+  # The concurrency group contains the workflow name and the branch name for
+  # pull requests or the commit hash for any other events.
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
+jobs:
+
+  linux:
+    name: Linux
+    uses: ./.github/workflows/ci-linux.yml
+    secrets: inherit
+
+  linux-asan:
+    name: Linux ASan
+    uses: ./.github/workflows/ci-linux_asan.yml
+    secrets: inherit
+
+  macos:
+    name: macOS
+    uses: ./.github/workflows/ci-macos.yml
+    secrets: inherit
+
+  windows:
+    name: Windows
+    uses: ./.github/workflows/ci-windows.yml
+    secrets: inherit