Continues #2676

• return to last position when opening file (defaults.vim)
• display=truncate
• pumblend=20 (for termguicolors/RGB only)
• in q: window: au CmdWinEnter [:>] syntax sync maxlines=1 minlines=1 (or some other low number)
  • feat(defaults): support for default autocommands #15401
• Auto-create 'backupdir' if 'backup' is set Backupdir default #12666
• hidden. Why: for 'nohidden' without persistent undo, hiding a buffer loses undo info.
  • defaults: enable 'hidden', disable 'joinspaces' #15410
• inoremap <C-U> <C-G>u<C-U>
• viewoptions-=options make :mkview not save options tpope/vim-sensible#164
• inccommand=nosplit feat: defaults: inccommand=nosplit #15395
• switchbuf=uselast
• sidescroll=1 https://ddrscott.github.io/blog/2016/sidescroll/ defaults: sidescroll=1 #7984
• background=dark [RFC] options: 'background' defaults to 'dark' #2894 defaults, part 1 #2676 (comment)
  • Already the default for "most cases", see :h 'bg'.
• sessionoptions+=unix,slash 1c3ca4f, 2c1d12d, session: restore same :terminal buf split windows #7836
• nnoremap <silent> <C-L> :nohlsearch<CR><C-L> defaults: make <C-L> clear search highlights and update diffs #15385
• :set shortmess+=F (statusline already has this info) defaults: shortmess+=F #8619
• enable 'cscopeverbose' defaults: enable 'cscopeverbose' (and deprecate it) #7888
• fillchars=vert:│,fold:· change defaults for fillchars 'vert' and 'fold' #7986
• wildoptions=tagfile,pum defaults: wildoptions=pum,tagfile #10384
• shortmess-=S defaults: exclude "S" from 'shortmess' #10136
• nostartofline defaults: set nostartofline #11135