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.
