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

runtime/lua/vim/lsp.lua

@@ -1534,8 +1534,5 @@ function lsp._with_extend(name, options, user_config)
   return resulting_config
 end
 
--- Define the LspDiagnostics signs if they're not defined already.
-require('vim.lsp.diagnostic')._define_default_signs_and_highlights()
-
 return lsp
 -- vim:sw=2 ts=2 et