wip: add vim language server (and hopefully grammar)

Closes #86
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";
+  };
+}