Align browser registerMarker alt-buffer guard with headless

Block registerMarker on the alternate buffer in CoreBrowserTerminal,
matching headless addMarker behavior. Update public typings to
IMarker | undefined and add regression tests.

Fixes #5937

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-search/src/DecorationManager.ts

@@ -131,6 +131,9 @@ export class DecorationManager extends Disposable {
     const decorations: IDecoration[] = [];
     for (const range of decorationRanges) {
       const marker = this._terminal.registerMarker(range[0]);
+      if (!marker) {
+        continue;
+      }
       const decoration = this._terminal.registerDecoration({
         marker,
         x: range[1],