Rename headless core addMarker to registerMarker

Align headless Terminal core API with browser CoreBrowserTerminal
and the public registerMarker naming.

Fixes #5938

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

src/headless/Terminal.ts

@@ -69,7 +69,7 @@ export class Terminal extends CoreTerminal {
     return this.buffer.markers;
   }
 
-  public addMarker(cursorYOffset: number): IMarker | undefined {
+  public registerMarker(cursorYOffset: number): IMarker | undefined {
     // Disallow markers on the alt buffer
     if (this.buffer !== this.buffers.normal) {
       return;

src/headless/public/Terminal.ts

@@ -141,7 +141,7 @@ export class Terminal extends Disposable implements ITerminalApi {
   }
   public registerMarker(cursorYOffset: number = 0): IMarker | undefined {
     this._verifyIntegers(cursorYOffset);
-    return this._core.addMarker(cursorYOffset);
+    return this._core.registerMarker(cursorYOffset);
   }
   public addMarker(cursorYOffset: number): IMarker | undefined {
     return this.registerMarker(cursorYOffset);