feat(health): checkhealth buffer can show in a split window (#26714)

:checkhealth now respects :vertical and :horizontal.
For example:
  :vertical checkhealth foo bar
will open the healthcheck buffer in a vertical split.

runtime/lua/vim/health.lua

@@ -268,14 +268,27 @@ end
 
 -- Runs the specified healthchecks.
 -- Runs all discovered healthchecks if plugin_names is empty.
-function M._check(plugin_names)
+-- splitmod controls how the healthcheck window opens: "vertical", "horizontal" or "tab"
+function M._check(splitmod, plugin_names)
   local healthchecks = plugin_names == '' and get_healthcheck('*') or get_healthcheck(plugin_names)
 
-  -- Create buffer and open in a tab, unless this is the default buffer when Nvim starts.
   local emptybuf = vim.fn.bufnr('$') == 1 and vim.fn.getline(1) == '' and 1 == vim.fn.line('$')
-  local mod = emptybuf and 'buffer' or 'tab sbuffer'
+  local mod = function()
+    if splitmod == 'vertical' then
+      return 'vertical sbuffer'
+    elseif splitmod == 'horizontal' then
+      return 'horizontal sbuffer'
+    elseif emptybuf then
+      -- if this is the default buffer when Nvim starts, open healthcheck directly
+      return 'buffer'
+    else
+      -- if not specified otherwise open healthcheck in a tab
+      return 'tab sbuffer'
+    end
+  end
+
   local bufnr = vim.api.nvim_create_buf(true, true)
-  vim.cmd(mod .. ' ' .. bufnr)
+  vim.cmd(mod() .. ' ' .. bufnr)
 
   if vim.fn.bufexists('health://') == 1 then
     vim.cmd.bwipe('health://')