treewide: rewrite autocompletion module and related stuff (#404)

* modules/completion: rewrite

* treewide: remove vsnip, add luasnip

* nvim-cmp: add default sorting

* nvim-cmp: load after luasnip

* lib: fix docs for mergelessListOf

* docs: add changelog entires for rewrite

* deprecations: add rewrite deprecations

* nvim-cmp: clarify in format description

* docs: fix option reference in release notes

* treewide: remove reduant `// {default = false;}`s

* luasnip: add missing `{option}` for option reference

* deprecations: add entry for vsnip

* nvim-autopairs: use multiline string

* nvim-dap: use outer attribute

modules/plugins/assistant/copilot/config.nix

@@ -34,6 +34,8 @@ in {
         "copilot-cmp"
       ];
 
+    vim.autocomplete.nvim-cmp.sources = {copilot = "[Copilot]";};
+
     vim.pluginRC.copilot = entryAnywhere ''
       require("copilot").setup(${toLuaObject cfg.setupOpts})