menu: add BOOT_KEYBIND for direct entry booting

[viewerofall]

Adds a global BOOT_KEYBIND: <key> <index> config option that binds a
single key to boot into the given entry based on its place in a flat list, not expanded meaning directories are just counted as another one so everything under them is just another one.

Behaviour

• Case-insensitive on both sides (config value and keypress), so H and
  h are equivalent.
• Respects keyboard_layout remapping — the bound key is matched against
  the post-remap character, same as every other menu keystroke.
• Cannot target a sub-menu/directory entry, only a bootable entry.
• Keys that are hardcoded or already bound such as (1-9, e, s, u, b) will make the keybind be skipped, giving the hardcoded ones priority
• If the same key is bound more than once, the one listed first wins.
• Any malformed line is skipped silently allowing you to have a successful go with your config, just a dysfunctional line.
• Index follows a flat list, not a tree one you do not have to expand directories in order to boot your entry.

Testing

• Built and booted under QEMU (BIOS and UEFI) against a minimal limine protocol setup over 3 entries
• Verified on real hardware (UEFI boot).
• Confirmed keybinds fire during the timeout countdown as well as after
  it's aborted (single keypress does both).

Docs

CONFIG.md updated with the new BOOT_KEYBIND option under the
Miscellaneous global options list.

---

Assisted-by: Claude:claude-sonnet-5

[viewerofall]

@Mintsuki when you are done with everything and such, can you review or comment then merge this. Its been a week and I dont want any code editing this making it not work. Thank you.