[feat] timeout 0 functionality like systemd-boot

[fxzzi]

According to the systemd-boot manual:

The boot menu is shown when a non-zero menu timeout has been
configured. If the menu timeout has been set to zero, hold down a
key (space is recommended) before the boot loader initializes to
bring up the boot menu. Note that depending on the firmware
implementation the time window where key presses are accepted
before the boot loader initializes might be short. If the window
is missed, reboot and try again, possibly repeatedly pressing a
suitable key; on most systems it should be possible to hit the
time window after a few attempts. Keys other than the space bar
may be used, except for the keys listed above. If showing the menu
on demand doesn't work well, consider setting a non-zero timeout
to show the boot menu unconditionally.

Under limine, setting a timeout of 0 completely hides the boot menu without any possibility of spamming space bar to show it. Setting timeout to 1 as well as quiet: yes gets most the way, but spamming space boots into the default entry straight away instead of just pausing at the menu. I don't think pressing space should boot into the selected entry. It's possible to use other keys but I think it would make sense to allow space bar.

Another feature on systemd-boot is being able to quickly select a boot entry just by the letter it starts with. This is really useful if, for example you use NixOS, have a bunch of boot entries, and have windows all the way on the bottom. On sysd-boot, I'd just press W on my keyboard to quickly jump to the entry, then enter to proceed.