Feature: remove tchar.h, bring the project code base to Unicode

[Demonese]

Welcome to Unicode!

There is a problem of mixing char and tchar in this project.

So I spent some time migrating it to Unicode.

[walbourn]

This codebase is ancient, so that's why TCHAR still was there... I appreciate the effort to clean it up.

[Demonese]

I ran all the GitHub actions in my fork and they all passed.

I think it's ready to be merge.

• CMake (Windows)
• CMake (Windows using VCPKG)
• Lint
• Microsoft C++ Code Analysis
• CodeQL