feat: Add support for accessing attached buffer number via subscribeAttach callback or LspointsAttach autocmd

[mityu]

This PR adds support for getting attached buffer number information while hooks invoked when LSP is attached to some buffer:

• Callbacks of subscribeAttach is fired with two arguments: client name and attached buffer number.
• LspointsAttach autocmd is fired in this form: LspointsAttach:{server-name}:{attached-buffer-number}

Please tell me if we should use another style for the accessing way of bufnr, e.g. give a utility function/variable to get attached bufnr information instead of changing LspointsAttach autocmd pattern.