CI and custom build script utilities

- macOS DMG image using AppDMG with custom background
- Linux AppImage
- Windows MSYS2 packaging
- Source code tarball including subprojects source code
- LHelper compatible build script

scripts/appdmg.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+set -ex
+
+if [ ! -e "src/api/api.h" ]; then
+  echo "Please run this script from the root directory of Lite XL."
+  exit 1
+fi
+
+cat > lite-xl-dmg.json << EOF
+{
+  "title": "Lite XL",
+  "icon": "$(pwd)/resources/icons/icon.icns",
+  "background": "$(pwd)/resources/macos/appdmg.png",
+  "window": {
+    "position": {
+      "x": 360,
+      "y": 360
+    },
+    "size": {
+      "width": 480,
+      "height": 360
+    }
+  },
+  "contents": [
+    { "x": 144, "y": 248, "type": "file", "path": "$(pwd)/Lite XL.app" },
+    { "x": 336, "y": 248, "type": "link", "path": "/Applications" }
+  ]
+}
+EOF
+~/node_modules/appdmg/bin/appdmg.js lite-xl-dmg.json "$(pwd)/$1.dmg"