feat(help): gx opens help tag in web browser (#35784)

Problem:
`gx` does not work on tags in help buffers to open the documentation of that tag in the browser.

Solution:
Get the `optionlink`, `taglink` and `tag` TS nodes and set extmark "url" property.
`gx` then discovers the extmark "url" and opens it.

Co-authored-by: Yochem van Rosmalen <git@yochem.nl>

test/functional/lua/ui_spec.lua

@@ -13,7 +13,7 @@ local poke_eventloop = n.poke_eventloop
 
 describe('vim.ui', function()
   before_each(function()
-    clear()
+    clear({ args_rm = { '-u' }, args = { '--clean' } })
   end)
 
   describe('select()', function()
@@ -180,5 +180,28 @@ describe('vim.ui', function()
         end, { n.testprg('printargs-test'), 'arg1' })
       )
     end)
+
+    it('gx on a help tag opens URL', function()
+      n.command('helptags $VIMRUNTIME/doc')
+      n.command('help nvim.txt')
+
+      local link_ns = n.api.nvim_create_namespace('nvim.help.urls')
+      local tag = n.api.nvim_buf_get_extmarks(0, link_ns, 0, -1, {
+        limit = 1,
+        details = true,
+      })[1]
+
+      local url = tag[4].url
+      assert(url)
+
+      --- points to the neovim.io site
+      eq(true, vim.startswith(url, 'https://neovim.io/doc'))
+
+      -- tag is URI encoded
+      local param = url:match('%?tag=(.*)')
+      local tagname =
+        n.api.nvim_buf_get_text(0, tag[2], tag[3], tag[4].end_row, tag[4].end_col, {})[1]
+      eq(vim.uri_encode(tagname), param)
+    end)
   end)
 end)