feat: NSIS Windows installer and update check (#32)

- windows/installer.nsi: installs to Program Files, manages system PATH,
  registers uninstaller
- relspec update [--check] [--yes]: compares against the latest release and
  prompts; on Windows downloads and runs the installer
- release workflow builds and uploads relspec-setup-windows-amd64.exe
- make installer-windows

.gitea/workflows/release.yml

@@ -78,6 +78,13 @@ jobs:
             echo "Built $NAME"
           done
 
+      - name: Build Windows installer
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y nsis
+          VERSION="${{ github.event.inputs.tag || github.ref_name }}"
+          makensis -DVERSION="${VERSION#v}" -DEXE="$PWD/relspec-windows-amd64.exe" -DOUT="$PWD/relspec-setup-windows-amd64.exe" windows/installer.nsi
+
       - name: Create release and upload assets
         run: |
           TAG="${{ github.event.inputs.tag || github.ref_name }}"