Refactor tmux and set-clipboard detection

Move logic to `autoload/tmux.vim` to detect the `vim` is being run
inside a `tmux` sessions and if the `set-clipboard` option is enabled.
This cleans up adding the optional `oscyank.vim` plugin when needed.

autoload/tmux.vim

@@ -1,5 +1,9 @@
 let s:tmux_option = '@vim'.substitute($TMUX_PANE, '%', '\\%', 'g')
 
+function! tmux#inSession() abort
+  return $TMUX !=# ''
+endfunction
+
 function! tmux#setNavigationFlag() abort
   call system('tmux set-window-option '.s:tmux_option.' 1')
 endfunction
@@ -7,3 +11,11 @@ endfunction
 function! tmux#unsetNavigationFlag() abort
   call system('tmux set-window-option -u '.s:tmux_option)
 endfunction
+
+function! tmux#isSetClipboardOn() abort
+  if !tmux#inSession()
+    return 0
+  endif
+  let l:set_clipboard = trim(system('tmux show-options -g set-clipboard'))
+  return l:set_clipboard ==# 'set-clipboard on'
+endfunction