[atom] Revert getLastCursor type (#34869)

Apparently, `getLastCursor` returning `undefined` was unintened and evidently was at some point silently fixed, at least in most cases.

types/atom/atom-tests.ts

@@ -17,7 +17,6 @@ declare const mouseEvent: MouseEvent;
 
 declare let buffer: Atom.TextBuffer;
 declare const color: Atom.Color;
-declare let maybeCursor: Atom.Cursor | undefined;
 declare let cursor: Atom.Cursor;
 declare let cursors: Atom.Cursor[];
 declare let decoration: Atom.Decoration;
@@ -2790,7 +2789,7 @@ function testTextEditor() {
     editor.moveToBeginningOfPreviousParagraph();
     editor.selectLargerSyntaxNode();
     editor.selectSmallerSyntaxNode();
-    maybeCursor = editor.getLastCursor();
+    cursor = editor.getLastCursor();
 
     str = editor.getWordUnderCursor();
     str = editor.getWordUnderCursor({});

types/atom/index.d.ts

@@ -1939,7 +1939,7 @@ export class TextEditor {
     moveToBeginningOfPreviousParagraph(): void;
 
     /** Returns the most recently added Cursor. */
-    getLastCursor(): Cursor | undefined;
+    getLastCursor(): Cursor;
 
     /** Returns the word surrounding the most recently added cursor. */
     getWordUnderCursor(options?: {