refactor: move init_default_autocmds to lua

The original motivation for this change came from developping
https://github.com/neovim/neovim/pull/22159, which will require adding
more autocommand creation to Neovim's startup sequence.

This change requires lightly editing a test that expected no autocommand
to have been created from lua.

test/functional/api/autocmd_spec.lua

@@ -209,7 +209,7 @@ describe('autocmd api', function()
 
       local aus = meths.get_autocmds({ event = 'User', pattern = 'Test' })
       local first = aus[1]
-      eq(first.id, 1)
+      eq(true, first.id > 0)
 
       meths.set_var("some_condition", true)
       meths.exec_autocmds("User", {pattern = "Test"})