feat(neovim): switch to null-ls

- switch from diagnostic-languageserver to null-ls.nvim
- add shellcheck & shfmt
- replace nix-linter with statix
- replace prettier with prettier_d_slim
- add stylelint, eslint_d & prettier_d_slim binaries

home/editors/neovim/default.nix

@@ -55,6 +55,7 @@ in {
 
       # lsp
       nvim-lspconfig
+      null-ls-nvim
       nvim-lightbulb
 
       # dap