feat(nvim): highlight mustache templates

nvim detects `.mustache` but ships no syntax for it, leaving Moodle templates
as plain text. `vim-mustache-handlebars` supplies syntax, indent and ftdetect,
covering the constructs the Handlebars tree-sitter grammar rejects: bare
sections, `{{{unescaped}}}`, partials and `{{< parent }}` inheritance.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015D4NsszUG7hy9ghKZ4nfhm

modules/nvim.nix

@@ -248,6 +248,17 @@ in
         };
       };
 
+      # Moodle/Mustache templates. nvim detects `.mustache` as filetype mustache
+      # but ships no syntax file for it, so the buffer renders as plain text.
+      # Tree-sitter is not an option here: the Handlebars grammar (glimmer)
+      # rejects mustache's argument-less sections, `{{{unescaped}}}`, partials
+      # and Moodle's `{{< parent }}` inheritance, and a single error node drops
+      # highlighting for the rest of the file. This plugin carries the syntax,
+      # indent and ftdetect scripts. nixpkgs marks it unfree only because
+      # upstream ships no license metadata; `allowUnfree` in modules/nixos.nix
+      # already covers it.
+      extraPlugins = [ pkgs.vimPlugins.vim-mustache-handlebars ];
+
       # Global (non-LSP) keymaps. Deliberately small — a starting point to grow.
       keymaps = [
         { mode = "n"; key = "<leader>w"; action = "<cmd>w<cr>"; options.desc = "Save"; }