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.

plugin/mappings.vim

@@ -77,7 +77,7 @@ nnoremap <leader>tm :tabmove<Space>
 " Clear search highlights
 nnoremap <leader><Space> :nohlsearch<CR>
 
-if g:use_osc52
+if exists('loaded_oscyank')
   noremap <leader>y :Oscyank<CR>
 else
   " System clipboard yank/put