Add CommonLibF4 submodule and build script

Adds CommonLibF4 as a git submodule dependency and introduces a Windows `build.bat` helper. The script ensures `xmake` is installed, runs the plugin build, and reports success/failure with the expected staged output path, making local setup and builds more consistent.

build.bat

@@ -0,0 +1,23 @@
+@echo off
+setlocal
+
+cd /d "%~dp0"
+
+where xmake >nul 2>&1
+if errorlevel 1 (
+    echo xmake was not found on PATH.
+    echo Install xmake 3.0.0 or newer from https://xmake.io
+    exit /b 1
+)
+
+echo Building F4SEPluginTemplate...
+xmake build
+if errorlevel 1 (
+    echo Build failed.
+    exit /b 1
+)
+
+echo.
+echo Build succeeded.
+echo Plugin staged to: package\F4SE\Plugins\
+exit /b 0