ci: check URL reachability #35593

Problem:
scripts/check_urls.vim manually matches urls in the help pages and then
synchronously checks them via curl/wget/powershell. This is extremely
slow (~5 minutes for Nvims runtime on my machine) and prone to errors in
how the urls are matched.

Solution:
- Use Tree-sitter to find the urls in the help pages and `vim.net.request` to
  check the responses.
- Add a `lintdocurls` build task and check it in CI (every Friday).
  - Reopens a dedicated issue if it finds unreachable URLs.
- Drop the old check_urls.vim script.

scripts/lintdoc.lua

@@ -16,7 +16,7 @@ print('Running lintdoc ...')
 vim.cmd [[ packadd netrw ]]
 vim.cmd [[ helptags ALL ]]
 
-require('src.gen.gen_help_html').run_validate()
+require('src.gen.gen_help_html').run_validate(nil, _G.arg[1] ~= nil)
 require('src.gen.gen_help_html').test_gen()
 
 print('lintdoc PASSED.')