feat: add is_first/is_last properties + expose buffers

[cwqt]

Adds is_first & is_last properties onto the buffer parameter. Useful for knowing if a tab is the first or last in the set of buffers.

Also exposes all the buffers as an argument, e.g.

{
  text: function(buffer, buffers) 
    if(buffer.is_first) return "first"
    if(buffer.is_last) return "last"
    return buffer.index .. "/" .. #buffers
  end
}

I've been using it in my config for a bit now https://gitlab.com/cxss/dotfiles/-/blob/main/nvim/lua/core/bufferline.lua#L52

[noib3]

There's no need to set these in every iteration of the loop, just set them once before the loop

_G.cokeline.visible_buffers[1].is_first = true
_G.cokeline.visible_buffers[#_G.cokeline.visible_buffers].is_first = true
for ...

[noib3]

What's the benefit of exposing all the valid buffers to the users?

[willothy]

Feel free to ping me if this is still needed, but since it doesn't seem necessary to expose all of the buffers and we already have #69 merged from a while ago, I'm going to close this.