Restore `:terminal` Buffers After Restart

Neovim embeds a VT220/xterm terminal emulator based on libvterm, which lets us work with the terminal right inside Neovim. However, `:terminal` buffers currently lose all their content when Neovim restarts, the buffer is closed, or a session is reloaded. My proposal aims to close this gap by preserving terminal history and bringing it back when users need it.