fix(provider): misleading :checkhealth if user sets g:loaded_python3_provider=1 #32696

Problem:
:checkhealth shows a confusing message if user sets
g:loaded_python3_provider=1.

Solution:
- Show a warning if that var is set to 1.
- Update provider modules to default to 0. Any user code that is
  checking for 1, is like already broken because these may be set to 2.

runtime/autoload/provider/python3.vim

@@ -11,5 +11,5 @@ function! provider#python3#Require(host) abort
 endfunction
 
 let s:prog = v:lua.vim.provider.python.detect_by_module('neovim')
-let g:loaded_python3_provider = empty(s:prog) ? 1 : 2
+let g:loaded_python3_provider = empty(s:prog) ? 0 : 2
 call v:lua.require'vim.provider.python'.start()