Extends the installer to write a GRUB chainload ESP layout when
bootloader = "grub" is set in config. Changes GeneralConfig, DiskOption,
fetch_bootloaders, with_whole_disk, with_whole_disk_ext4, and both CLI
binaries. When GRUB mode is active, the ESP contains GRUB as primary
(BOOTX64.EFI), grub.cfg, and the Redox bootloader as chainload target
(EFI/REDBEAR/redbear.efi).
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
vasilito
9a58b786ce