feat(diagnostics): Add support for nvims diagnostic API

- neovim added new diagnostics api in https://github.com/neovim/neovim/pull/15585.
  So I've added a new diagnostics source named `nvim` (Yes I couldn't
  find a name for it :P) that shows diagnostics from that api.
  In neovim nightly with vim.diagnostics you can consider source
  `nvim_lsp` obsolete.
- Diagnostics source `vim_lsp` was removed in a mistake now it's
  restored.

README.md

@@ -366,7 +366,7 @@ sections = {
     {
       'diagnostics',
       -- table of diagnostic sources, available sources:
-      -- 'nvim_lsp', 'coc', 'ale', 'vim_lsp'
+      -- 'nvim_lsp', 'nvim', 'coc', 'ale', 'vim_lsp'
       -- Or a function that returns a table like
       --   {error=error_cnt, warn=warn_cnt, info=info_cnt, hint=hint_cnt}
       sources = {},

doc/lualine.txt

@@ -349,7 +349,7 @@ Component specific local options ~
         {
           'diagnostics',
           -- table of diagnostic sources, available sources:
-          -- 'nvim_lsp', 'coc', 'ale', 'vim_lsp'
+          -- 'nvim_lsp', 'nvim', 'coc', 'ale', 'vim_lsp'
           -- Or a function that returns a table like
           --   {error=error_cnt, warn=warn_cnt, info=info_cnt, hint=hint_cnt}
           sources = {},

lua/lualine/components/diagnostics.lua

@@ -183,6 +183,17 @@ Diagnostics.diagnostic_sources = {
     local hint_count = vim.lsp.diagnostic.get_count(0, 'Hint')
     return error_count, warning_count, info_count, hint_count
   end,
+  nvim = function()
+    local diagnostics = vim.diagnostic.get(0)
+    local count = { 0, 0, 0, 0 }
+    for _, diagnostic in ipairs(diagnostics) do
+      count[diagnostic.severity] = count[diagnostic.severity] + 1
+    end
+    return count[vim.diagnostic.severity.ERROR],
+      count[vim.diagnostic.severity.WARN],
+      count[vim.diagnostic.severity.INFO],
+      count[vim.diagnostic.severity.HINT]
+  end,
   coc = function()
     local data = vim.b.coc_diagnostic_info
     if data then
@@ -199,6 +210,14 @@ Diagnostics.diagnostic_sources = {
       return 0, 0, 0, 0
     end
   end,
+  vim_lsp = function()
+    local ok, data = pcall(vim.fn['lsp#get_buffer_diagnostics_counts'])
+    if ok then
+      return data.error, data.warning, data.information
+    else
+      return 0, 0, 0
+    end
+  end,
 }
 
 Diagnostics.get_diagnostics = function(sources)