Error out if git symlink not enabled on Windows (#15684)

When I install executorch on windows I run into this error:   error:
[Errno 2] No such file or directory:
'pip-out\\lib.win-amd64-cpython-312\\executorch\\version.py'

This is because we are not enabling symlink in git.

Need to run:

```
 git config --global core.symlinks true
```

and reclone the repo.

This PR fails the installation when that happens.

Author: larryliu0820

install_executorch.bat

@@ -1,4 +1,5 @@
 @ECHO OFF
+setlocal EnableDelayedExpansion
 
 rem Copyright (c) Meta Platforms, Inc. and affiliates.
 rem All rights reserved.
@@ -7,9 +8,24 @@ rem This batch file provides a basic functionality similar to the bash script.
 
 cd /d "%~dp0"
 
+rem Verify that Git checked out symlinks correctly. Without this the Python install
+rem will fail when attempting to copy files from src\executorch.
+where git >NUL 2>&1
+if not errorlevel 1 (
+    set "GIT_SYMLINKS="
+    for /f "usebackq delims=" %%i in (`git config --get core.symlinks 2^>nul`) do set "GIT_SYMLINKS=%%i"
+    if /I not "!GIT_SYMLINKS!"=="true" (
+        echo ExecuTorch requires Git symlink support on Windows.
+        echo Enable Developer Mode and run: git config --global core.symlinks true
+        echo Re-clone the repository after enabling symlinks, then rerun install_executorch.bat.
+        exit /b 1
+    )
+)
+
 rem Under windows, it's always python
 set PYTHON_EXECUTABLE=python
 
 "%PYTHON_EXECUTABLE%" install_executorch.py %*
 
-exit /b %ERRORLEVEL%
+set "EXIT_CODE=%ERRORLEVEL%"
+endlocal & exit /b %EXIT_CODE%