Experimental: map S-Tab to delete one indent

Seems friendly (similar to vscode and other editors), harmless (doesn't
seem to break anything as S-Tab normally acts same as Tab), and plays
nicely with visual mode S-Tab mapping to shift selection leftwards.

Idea from @samjwill, see tpope/vim-sensible#173

FIXME: make pumvisible() aware expr map trigger any existing S-Tab map

plugin/friendly.vim

@@ -269,6 +269,11 @@ vnoremap < <gv
 vnoremap <Tab> >gv
 vnoremap <S-Tab> <gv
 
+" Allow Shift+Tab to delete one shiftwidth of indent (aka "outdent")
+" Seems friendly (similar to vscode and some other editors), and harmless
+" (Shift+Tab is not mapped by default in insert mode, does same as Tab)
+inoremap <S-Tab> <C-d>
+
 " Allow basic windows style editing shortcuts. Copied from mswin.vim but
 " implements fewer shortcuts, in fewer modes, to avoid breaking things like
 " CTRL-A/CTRL-X to add/subtract numbers and CTRL-V for visual block mode.
@@ -442,7 +447,8 @@ function! <SID>SimpleTab()
   endif
 endfunction
 inoremap <expr> <Tab> pumvisible() ? "\<C-n>" : <SID>SimpleTab()
-inoremap <expr> <S-Tab> pumvisible() ? "\<C-p>" : "\<S-Tab>"
+" FIXME: remove duplication, trigger existing S-Tab map when pum not visible
+inoremap <expr> <S-Tab> pumvisible() ? "\<C-p>" : "\<C-d>"
 " Experimental: map CR to accept completion, should be safe enough assuming
 " friendly.vim is loaded before other plugins which may otherwise map CR.
 " Some plugins might skip adding their own CR mapping if one exists, which