Remove the .tests/vendor/plenary.nvim git submodule so Neovim plugin
managers (lazy.nvim, etc.) don't pull it during installation. Instead,
run_tests.ps1 and run_checks.ps1 clone plenary.nvim on demand if it's
not present. The .gitea CI workflow also gets its own bootstrap step.
Add .gitignore to exclude the bootstrapped vendor directory.
