wip: add vim language server (and hopefully grammar)

We have the language server working, but no luck yet on the grammar.
May require PRing into Helix to add it, but it'd be difficult to get
working if I can't have it working on my machine.

apps/cli/helix/vim.nix

@@ -0,0 +1,42 @@
+{ pkgs, lib, ... }:
+
+let
+  url = "https://github.com/tree-sitter-grammars/tree-sitter-vim";
+  rev = "f3cd62d8bd043ef20507e84bb6b4b53731ccf3a7";
+in
+{
+  hm.programs.helix.languages.grammar = lib.singleton
+  {
+    name = "vim";
+    source =
+    {
+      git = url;
+      inherit rev;
+    };
+  };
+
+  hm.programs.helix.languages.language = lib.singleton
+  {
+    name = "vim";
+    scope = "source.vim";
+    file-types =
+    [
+      "vim"
+      {
+        glob = ".vimrc";
+      }
+    ];
+    roots = lib.singleton "addon-info.json";
+    comment-token = "\"";
+    language-servers = lib.singleton
+    {
+      name = "vim-language-server";
+    };
+  };
+
+  hm.programs.helix.languages.language-server.vim-language-server =
+  {
+    command = lib.getExe pkgs.vim-language-server;
+    args = lib.singleton "--stdio";
+  };
+}