fix(help): :helptags regressions

Problem:
Parent commit regressed some behavior of the old C helptags-gen impl:
- helpfiles in sub-directories are named by basename, so :help fails
- "help-tags" always names "tags", never "tags-nl"
- E150 E151 E152 E153 are never reported
- existing tags file is not overwritten if no tags were found
- a duplicate tag aborts the run, skipping the remaining directories
- `*.TXT` and `*.FRX` (uppercase) are not recognized as helpfiles
- tree-sitter-vimdoc accepts tags that the C parser rejected: `*a|b*`
  (breaks |links|) and unterminated `*tag`
- PUC Lua "<" compares with (localized) `strcoll()`, so the tags file is
  not sorted by byte value (E432)
- requiring `vim.treesitter` at load time breaks :help itself, not just
  :helptags, where the module is unavailable
- `vim.pack` runs :helptags for plugins that have no "doc/" directory,
  so every install/update emits E150

Solution:
- Fail the build if generating helptags reports any `v:errmsg`.
- Restore old behavior: tags generated for runtime/doc are now identical
  to those from the C implementation. Errors are non-fatal messages
  instead of exceptions, so all directories are still processed.
- Load treesitter lazily, report a plain error if parser is missing.

runtime/lua/vim/pack.lua

@@ -835,11 +835,12 @@ local function checkout(p, timestamp, skip_stash)
   plugin_lock.plugins[p.spec.name].rev = p.info.sha_target
 
   -- (Re)Generate help tags according to the current help files.
-  -- Also use `pcall()` because `:helptags` errors if there is no 'doc/'
-  -- directory or if it is empty.
+  -- Also use `pcall()` because `:helptags` errors if 'doc/' has no help files.
   local doc_dir = vim.fs.joinpath(p.path, 'doc')
   vim.fn.delete(vim.fs.joinpath(doc_dir, 'tags'))
-  copcall(vim.cmd.helptags, { doc_dir, magic = { file = false } })
+  if vim.fn.isdirectory(doc_dir) == 1 then
+    copcall(vim.cmd.helptags, { doc_dir, magic = { file = false } })
+  end
 end
 
 --- @param plug_list vim.pack.Plug[]