Fix: deprecation warning for Neovim healthcheck

[pappasam]

The current health check code is using deprecated functions that will be removed in Neovim 0.11.

checkhealth Output before PR

mkdp: health#mkdp#check

- Platform: linux
- WARNING health#report_info is deprecated, use vim.health.info instead. :help |deprecated|
  This feature will be removed in Nvim version 0.11
- Nvim Version: NVIM v0.10.0-dev-4eea609
- Node version: v18.7.0
  
- Script: .../markdown-preview.nvim/app/server.js
- Script exists: 1
- OK Using node
- WARNING health#report_ok is deprecated, use vim.health.ok instead. :help |deprecated|
  This feature will be removed in Nvim version 0.11

checkhealth Output after PR

mkdp: health#mkdp#check

- Platform: linux
- Nvim Version: NVIM v0.10.0-dev-4eea609
  Build type: RelWithDebInfo
  LuaJIT 2.1.1696795921
  Run "nvim -V1 -v" for more info
- Node version: v18.7.0
- Script: .../markdown-preview.nvim/app/server.js
- Script exists: 1
- OK Using node