Feat: Optimize counting of diagnostics with new vim.diagnostic.count() (nvim 0.10+)

[camoz]

Requested feature

Optimize counting of diagnostics with new function vim.diagnostic.count() (nvim 0.10+) where possible.

This is more efficient and thus preferred over using vim.diagnostic.get() for counting diagnostics manually (as it is currently done in lualine), according to https://neovim.io/doc/user/news-0.10.html

Motivation

Performance improvement.