fix: termopen previewer (#1901)

- buffer leaking
- still insert mode on confirm

doc/telescope.txt

@@ -2932,9 +2932,6 @@ previewers.new_termopen_previewer() *telescope.previewers.new_termopen_previewer
     Furthermore, it will forward all `config.set_env` environment variables to
     that terminal process.
 
-    While this interface is a good start, it was replaced with the way more
-    flexible `buffer_previewer` and is now deprecated.
-
 
 
 previewers.cat()                                  *telescope.previewers.cat()*