.gitignore rules matching `core` cause incorrect behavior on Windows

Summary

The .gitignore file contains the entry:

core

On Windows, this erroneously matches: eigen\Eigen\Core eigen\Eigen\src\Core

This messes up git tracking, but also in Visual Studio the Folder View of the CMake project hides these items.

Environment

• Operating System : Windows
• Architecture : x64
• Eigen Version : 3.4.0
• Compiler Version : Visual Studio 2022 17.5.3

Anything else that might help

• Have a plan to fix this issue.