feat: ALT key support in PS/2 keyboard driver

feat: ALT key support in PS/2 keyboard driver

Added alt_held state tracking for Left ALT (scancode 0x38/0xB8)
and Right ALT (E0 0x38/0xB8). When ALT is held, the driver
emits ESC (0x1B) prefix before the character, which is the
standard terminal encoding for Alt+key combinations.

Also handles Right CTRL (E0 0x1D/0x9D) which was previously
ignored since it uses E0-prefixed scancodes.

Summary of modifier keys now supported:
  - Shift (L/R): uppercase and symbol characters
  - CTRL (L/R):  control characters (c & 0x1F)
  - ALT (L/R):   ESC prefix + character