refactor: move vim.lsp.diagnostic to vim.diagnostic

This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).

More discussion at [1].

[1]: https://github.com/neovim/neovim/pull/15585

src/nvim/lua/vim.lua

@@ -105,6 +105,9 @@ setmetatable(vim, {
     elseif key == 'highlight' then
       t.highlight = require('vim.highlight')
       return t.highlight
+    elseif key == 'diagnostic' then
+      t.diagnostic = require('vim.diagnostic')
+      return t.diagnostic
     end
   end
 })