plugins.lsp: alias onAttach to new lsp.onAttach

This simplifies the impl by doing global on-attach logic in a
`LspAttach` autocmd instead of adding lua lines to each server's
individual `on_attach` callback.

This is effectively a `mkAliasOptionModule` alias, other than the alias
only being applied when `plugins.lsp.enable`.

plugins/by-name/rust-tools/default.nix

@@ -173,7 +173,6 @@ in
           server = {
             inherit (cfg.server) standalone;
             settings.rust-analyzer = lib.filterAttrs (n: v: n != "standalone") cfg.server;
-            on_attach = lib.nixvim.mkRaw "__lspOnAttach";
           };
         } // cfg.extraOptions;
       in