Use a prompt buffer for adding watches

This very new feature makes it kind of nice to add new watches in the
place where they will appear, keeping the eye in the same screen
location.

autoload/vimspector.vim

@@ -83,6 +83,12 @@ function! vimspector#AddWatch( expr ) abort
   py3 _vimspector_session.AddWatch( vim.eval( 'a:expr' ) )
 endfunction
 
+function! vimspector#AddWatchPrompt( expr ) abort
+  stopinsert
+  setlocal nomodified
+  call vimspector#AddWatch( a:expr )
+endfunction
+
 function! vimspector#ShowOutput( category ) abort
   py3 _vimspector_session.ShowOutput( vim.eval( 'a:category' ) )
 endfunction