KIND OF handle mode 3 (132 col mode)

This gets vttest looking slightly better on test 1, page 4. It still
isn't correct but we're looking better.

src/Window.zig

@@ -773,6 +773,23 @@ pub fn setMode(self: *Window, mode: terminal.Mode, enabled: bool) !void {
 
         .bracketed_paste => self.bracketed_paste = true,
 
+        .enable_mode_3 => self.terminal.modes.enable_mode_3 = @boolToInt(enabled),
+        .@"132_column" => mode3: {
+            // TODO: test this
+
+            // Do nothing if "enable mode 3" is not set.
+            if (self.terminal.modes.enable_mode_3 == 0) break :mode3;
+
+            // Set it
+            self.terminal.modes.@"132_column" = @boolToInt(enabled);
+
+            // TODO: do not clear screen flag mode
+            self.terminal.eraseDisplay(.complete);
+            self.terminal.setCursorPos(1, 1);
+
+            // TODO: left/right margins
+        },
+
         else => if (enabled) log.warn("unimplemented mode: {}", .{mode}),
     }
 }

src/terminal/Terminal.zig

@@ -47,6 +47,9 @@ modes: packed struct {
     reverse_colors: u1 = 0, // 5,
     origin: u1 = 0, // 6
     autowrap: u1 = 1, // 7
+
+    @"132_column": u1 = 0, // 3,
+    enable_mode_3: u1 = 0, // 40
 } = .{},
 
 /// Scrolling region is the area of the screen designated where scrolling