Added: Add SCROLL extra key to toggle auto scrolling of terminal to bottom on terminal text updates and termux activity return

The toggle will apply to each terminal session separately.

Closes #2535

termux-shared/src/main/java/com/termux/shared/termux/extrakeys/ExtraKeysConstants.java

@@ -90,6 +90,7 @@ public class ExtraKeysConstants {
             put("DRAWER", "☰"); // U+2630 ☰ TRIGRAM FOR HEAVEN not well known but easy to understand
             put("KEYBOARD", "⌨"); // U+2328 ⌨ KEYBOARD not well known but easy to understand
             put("PASTE", "⎘"); // U+2398
+            put("SCROLL", "⇳"); // U+21F3
         }};
 
         public static final ExtraKeyDisplayMap LESS_KNOWN_CHARACTERS_DISPLAY = new ExtraKeyDisplayMap() {{