[core] Consolidate setup scripts into a cross-platform setup.py

script/setup and script/setup.bat implemented the same workflow and were
kept in sync by hand, which had already drifted: the worktree hook fix in
#18843 landed in the bash script only. Both are now thin wrappers around
script/setup.py, so there is one implementation to maintain.

The post-checkout hook also checks for a venv/Scripts layout before doing
anything, so a branch switch on Windows can no longer clear a working
virtual environment.

script/setup.bat

@@ -1,28 +1 @@
-@echo off
-
-if defined VIRTUAL_ENV goto :install
-
-echo Starting the Virtual Environment
-python -m venv venv
-call venv/Scripts/activate
-echo Running the Virtual Environment
-
-:install
-
-echo Installing required packages...
-
-python.exe -m pip install --upgrade pip
-
-pip3 install -r requirements.txt -r requirements_test.txt -r requirements_dev.txt
-pip3 install setuptools wheel
-pip3 install -e ".[dev,test]" --config-settings editable_mode=compat
-
-rem --overwrite replaces any hook already in place. Without it, prek finds a
-rem previously installed pre-commit hook, moves it aside to
-rem .git/hooks/pre-commit.legacy and keeps calling it, so every commit would
-rem run both tools.
-prek install --overwrite
-
-echo .
-echo .
-echo Virtual environment created. Run 'venv/Scripts/activate' to use it.
+@python "%~dp0setup.py" %*