nvim: minor fixes for lilypond plugin

.config/nvim/after/ftplugin/lilypond.lua

@@ -54,4 +54,8 @@ if vim.b.first_start_lilypond then
 end
 vim.b.first_start_lilypond = true
 
+vim.schedule(function()
+  if vim.fn.exists(":Viewer") == 2 then
     vim.cmd.Viewer()
+  end
+end)

.config/nvim/lua/plugins/nvim-lilypond-suite.lua

@@ -1,6 +1,7 @@
 return {
   -- lillypond integration
   "martineausimon/nvim-lilypond-suite",
+  ft = "lilypond",
   dependencies = "uga-rosa/cmp-dictionary",
   config = function()
     require("nvls").setup({