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/screen/ea24.interop

@@ -0,0 +1,2 @@
+cc65's C64 runtime calls $EA24 (UPDCRAMPTR in its cbm_kernal.inc) after PLOT has set the cursor,
+for USER to follow PNT, before it stores a character's colour through USER.