feat: make resolved ProcessPromise iterable

Add a synchronous iterator to the `ProcessOutput` class to allow iterating over stdout lines.

* **src/core.ts**
  - Add a synchronous iterator method `[Symbol.iterator]()` to the `ProcessOutput` class.
  - Split the `stdout` string into lines using `\r?\n` to handle both Unix and Windows line endings.
  - Return the iterator of the split array.

* **test/core.test.js**
  - Add a test case to verify the synchronous iterator method in the `ProcessOutput` class.
  - Ensure the test case covers both Unix and Windows line endings.

src/core.ts

@@ -802,6 +802,13 @@ export class ProcessOutput extends Error {
   duration: ${this.duration}
 }`
   }
+
+  /**
+   * Synchronous iterator to allow iterating over stdout lines.
+   */
+  [Symbol.iterator](): IterableIterator<string> {
+    return this.stdout.split(/\r?\n/)[Symbol.iterator]();
+  }
 }
 
 export function usePowerShell() {

test/core.test.js

@@ -1161,6 +1161,12 @@ describe('core', () => {
         assert.match(ProcessOutput.getErrorMessage({}, ''), /Unknown error/)
       })
     })
+
+    test('synchronous iterator', () => {
+      const output = new ProcessOutput(0, null, 'line1\nline2\r\nline3\n', '', '', '');
+      const lines = Array.from(output);
+      assert.deepEqual(lines, ['line1', 'line2', 'line3']);
+    });
   })
 
   describe('cd()', () => {