feat(pack): hint in confirmation buffer that plugin is not active

Problem: After `vim.pack.update()` it is not clear if plugin is active
  or not. This can be useful to detect cases when plugin was removed
  from 'init.lua' but there was no `vim.pack.del()`.

Solution: Add ` (not active)` suffix with distinctive highlighting to
  header of plugins that are not active.
  It will also be shown in in-process LSP document symbols to have quick
  reference about which plugins are not active.

runtime/ftplugin/nvim-pack.lua

@@ -25,8 +25,10 @@ for i, l in ipairs(lines) do
     cur_header_hl_group = header_hl_groups[cur_group]
     hi_range(i, 0, l:len(), cur_header_hl_group)
   elseif l:find('^## (.+)$') ~= nil then
-    -- Header 2
+    -- Header 2 with possibly "(not active)" suffix
     hi_range(i, 0, l:len(), cur_header_hl_group)
+    local col = l:match('() %(not active%)$') or l:len()
+    hi_range(i, col, l:len(), 'DiagnosticError', priority + 1)
   elseif cur_info ~= nil then
     -- Plugin info
     local end_col = l:match('(). +%b()$') or l:len()