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.

test/functional/core/main_spec.lua

@@ -228,6 +228,7 @@ describe('vim._core', function()
       'vim._core.help',
       'vim._core.log',
       'vim._core.options',
+      'vim._core.proc',
       'vim._core.server',
       'vim._core.shared',
       'vim._core.spell',