feat: add `vim.diagnostic.open_float`

[Marc-AnthonyG]

Close #1375

I choose xl as the keymap because it is "near" my trouble.nvim keymap that also deal with diagnostic. I am all ear if you have other suggestion.

[DanWlker]

I think the existing keymap is <c-w>d

[dithmer]

I think the existing keymap is <c-w>d

You are absolutely right. Evidence in neovim's source can be found here: https://github.com/neovim/neovim/blob/master/runtime/lua/vim/_defaults.lua#L263

[OdysseusOperator]

How would i read this <c-w>d ? control & W followed by d ?