feat(health): report ulimit info

Problem:
Nvim shows `(libuv) kqueue(): Too many open files` on macos.
ref https://github.com/neovim/neovim/issues/40238

Solution:
Add a healthcheck for this situation.

runtime/lua/vim/_core/proc.lua

@@ -0,0 +1,20 @@
+-- OS/process utils.
+
+local M = {}
+
+--- Counts (approximate) open file descriptors for this process. Works on Linux/macOS/BSD.
+---@return integer? # Number of open fds, or nil if `/dev/fd` failed (fds exhausted, or unavailable/Windows).
+function M.count_open_fds()
+  local n = 0
+  for _, _, err in vim.fs.dir('/dev/fd', { err = true }) do
+    -- If `/dev/fd` scan failed (e.g. EMFILE when fds exhausted), count would be misleading.
+    if err then
+      return nil
+    end
+    n = n + 1
+  end
+  -- Discount the scan's own descriptor.
+  return math.max(0, n - 1)
+end
+
+return M