fix: wait until we enter a package.json file to register a pkg manager

[mwojick]

I call setup on nvim startup, so __register_package_manager would always return early because of this check:

    -- If we're not in a package.json file, exit
    if vim.fn.expand("%:t") ~= "package.json" then
        return
    end

similar to the other register fns, this waits until we actually enter a package.json file

[vuki656]

Thanks