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.

vimrc

@@ -200,9 +200,7 @@ Pack 'jrozner/vim-antlr'
 
 " Plugins for remote integrations
 Pack 'greymd/oscyank.vim', {'type': 'opt'}
-let g:use_osc52 = $TMUX !=# '' &&
-\ trim(system('tmux show-options -g set-clipboard')) ==# 'set-clipboard on'
-if g:use_osc52 && isdirectory(expand('~/.vim/pack/minpac/opt/oscyank.vim'))
+if tmux#isSetClipboardOn()
   packadd oscyank.vim
 endif