Lib: Improve IDLE launcher script for clarity and safety

[TheAhumMaitra]

This PR refactors the IDLE launcher script (IDLE.py) to improve clarity and maintainability.

Changes include:

• Simplified the way idlelib is added to sys.path for non-standard locations.
• Added comments explaining the purpose of each step.
• Ensured safer insertion into sys.path without overwriting existing paths.
• Made the script easier to read and understand for contributors and beginners.

This helps prevent errors when running IDLE from development versions of Python and makes the codebase more maintainable.

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[picnixz]

• This doesn't improve safety in any way.
• It's a purely cosmetic change. There is only one change that we could accept, that is putting main() inside an if __name__ == '__main__' guard but I'm not sure about its intended behavior (doing this would prevent loading IDLE by simply importing it, so it's a behavioral change). This should be discussed in an issue if needed but I don't think we need to.

[terryjreedy]

I agree with closure. With changes to 3 unrelated files, I wonder if this is llm spam.

I am not sure if the path insertion is needed and have considered removing it. It may be duplicated elsewhere. In the absence of a valid problem report, I will not touch it until I review all of the path manipulations in idlelib.

I routinely start IDLE from a freshly built Python REPL with import idlelib.idle, so the main change is WRONG.