feat: create Lean.gitignore

[quantum9Innovation]

Reasons for making this change

This adds a standard gitignore template for the Lean programming language.

Links to documentation supporting these rule changes

This page describes how Lake, Lean's package manager, works. Currently the template only ignores .lake which is where cache and other data is stored. Others are welcome to add/suggest other useful things to add to the gitignore template, though most Lean projects typically only have this in their gitignore.

If this is a new template

Lean project homepage: https://lean-lang.org/

Merge and Approval Steps

• Confirm that you've read the contribution guidelines and ensured your PR aligns
• Ensure CI is passing
• Get a review and Approval from one of the maintainers

[Copilot]

Pull request overview

Adds a new .gitignore template for Lean projects to ignore Lake build artifacts, aligning with how Lean’s Lake tool stores cache/build data.

Changes:

• Introduce Lean.gitignore with rules to ignore Lake’s .lake directory.
• Add a brief header comment describing the ignored path.

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