LibVT+Everywhere: Introduce 'automarks' and 'clear previous command'

Automarks are similar to bookmarks placed by the terminal, allowing the
user to selectively remove a single command and its output from the
terminal scrollback.
This commit implements a single way to add marks: automatically placing
them when the shell becomes interactive.

To make sure the shell behaves correctly after its expected prompt
position changes, the terminal layer forces a resize event to be passed
to the shell on such (possibly) partial clears; this also has the nice
side effect of fixing the disappearing prompt on the preexisting "clear
including history" action: Fixes #4192.

Userland/Applications/TerminalSettings/TerminalSettingsMain.gml

@@ -48,4 +48,22 @@
             text: "Confirm exit when process is active"
         }
     }
+
+    @GUI::GroupBox {
+        title: "Auto-mark behavior"
+        preferred_height: "fit"
+        layout: @GUI::VerticalBoxLayout {
+            margins: [8]
+        }
+
+        @GUI::RadioButton {
+            name: "automark_off"
+            text: "Do not auto-mark"
+        }
+
+        @GUI::RadioButton {
+            name: "automark_on_interactive_prompt"
+            text: "Auto-mark on interactive shell prompts"
+        }
+    }
 }