GHA: add initial workflow for building the package in CI

Introduce CI coverage for the package across all the major platforms.
This should ensure that we do not accidentally regress the package on
various platforms.

Author: compnerd

.github/workflows/build.yml

@@ -0,0 +1,53 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            swift-version: swift-6.1.2-release
+            swift-build: 6.1.2-RELEASE
+            build-args: "--traits GNU"
+
+          - os: macos-latest
+            swift-version: swift-6.1.2-release
+            swift-build: 6.1.2-RELEASE
+            build-args: ""
+
+          - os: windows-latest
+            swift-version: swift-6.1.2-release
+            swift-build: 6.1.2-RELEASE
+            build-args: ""
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Swift
+        uses: compnerd/gha-setup-swift@main
+        with:
+          swift-version: ${{ matrix.swift-version }}
+          swift-build: ${{ matrix.swift-build }}
+          update-sdk-modules: true
+
+      - if: matrix.os == 'ubuntu-latest'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libunistring-dev
+
+      - name: Show Swift version
+        run: swift --version
+
+      - name: Resolve dependencies
+        run: swift package resolve
+
+      - name: Build
+        run: swift build --configuration debug ${{ matrix.build-args }}