feat(kernal): $E5B4, $EA24, and $ECF0 at their well-known addresses

cc65's C64 runtime and ip65 call past the jump table: $E5B4 takes the first
key from the keyboard buffer (cgetc), $EA24 points USER at the colour RAM of
PNT's line (cursor positioning), and $ECF0 holds the low bytes of the screen
lines' addresses (ip65's VT100 code), as Mapping the Commodore 64 describes
them. vicii_init's work moves out of $E5A0-$E5C8 to free $E5B4, and on the
standard layout PNT and USER are worked out through the routine at $EA24.

src/kernal/keyboard/e5b4.get_from_keyboard_buffer.s

@@ -0,0 +1,21 @@
+;; #LAYOUT# STD *        #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+;
+; Well-known Kernal routine, described in:
+;
+; - [CM64] Computes Mapping the Commodore 64 - $E5B4, LP2
+;
+; Takes the first key from the keyboard buffer, which must not be empty, and moves the
+; others up. Out: the key in A. X is kept.
+;
+
+
+get_from_keyboard_buffer:
+
+	lda KEYD
+	pha
+	jsr pop_keyboard_buffer
+	pla
+
+	rts