feat: Migrate Vim and Tmux plugins to dedicated managers

Migrated Vim plugins from Git submodules to vim-plug, and Tmux plugins
to TPM (Tmux Plugin Manager).

This change:
- Updates `install.sh` to automatically install vim-plug and TPM.
- Modifies `dotfiles/vimrc` to configure plugins via vim-plug.
- Modifies `dotfiles/tmux.conf` to configure plugins via TPM.
- Removes all Vim and Tmux submodule entries from `.gitmodules`.
- Deletes the old submodule directories from the repository.

This streamlines plugin management, making updates easier and reducing
the complexity of the main `install.sh` script.

dotfiles/vimrc

@@ -9,6 +9,17 @@ if !isdirectory($HOME . '/.cache/vim/swap')
   silent !mkdir -p ~/.cache/vim/{backup,swap,undo}
 endif
 
+" vim-plug plugin list
+call plug#begin('~/.vim/plugged')
+
+Plug 'lifepillar/vim-solarized8'
+Plug 'tpope/vim-surround'
+Plug 'editorconfig/editorconfig-vim'
+Plug 'tpope/vim-fugitive'
+Plug 'ctrlpvim/ctrlp.vim'
+
+call plug#end()
+
 " Make sure files get completely written, but don't write backups
 set nobackup
 set writebackup