feat: add type annotations

[delphinus]

I'm adding type annotations and some documentations for plenary. At first, I have added to plenary.path because this is the most popular module here, I think.

TODO

• [x] plenary
• plenary.bit (bit already exists in LuaJIT) 
• [x] plenary.async
  • plenary.async.uv should be generated by scripts.
  • plenary.async.async should also be generated 🤔
• plenary.async_lib (deprecated)
• [x] plenary.class
• [x] plenary.context_manager
• [x] plenary.curl
• [x] plenary.filetype
• [x] plenary.fun
• [x] plenary.functional
• [x] plenary.iterators
• [x] plenary.job
• [x] plenary.log
• [x] plenary.nvim_meta
• [x] plenary.operators
• [x] plenary.path
• [x] plenary.reload
• [x] plenary.scandir
• [x] plenary.strings
• [ ] plenary.test_harness
• [x] plenary.window

[mochaaP]

Consider using type stubs from https://github.com/LuaCATS/busted and https://github.com/LuaCATS/luassert