Fix moveBack() history pointer + add symmetric moveForward()

[TavoNiievez]

1. Fix moveBack() to move the history pointer instead of appending

moveBack() replays the previous request via _loadPage(), which called clientRequest() with the default $changeHistory = true. Replaying with changeHistory = true appends the request to the BrowserKit history and discards the forward entries, so moveBack() didn't behave like a real Back button:

• the history pointer ended back on the last page after going back, and
• a second moveBack() no longer walked further back.

Demonstration (fails on master):

$I->amOnPage('/iframe');
$I->amOnPage('/info');
$I->amOnPage('/');
$I->moveBack();
$I->seeCurrentUrlEquals('/info'); // ok
$I->moveBack();
$I->seeCurrentUrlEquals('/iframe'); // FAILS on master -> lands on '/info'

Fix: thread a $changeHistory flag through _loadPage() and pass false from moveBack(). Matches Symfony's native AbstractBrowser::back() (requestFromRequest($request, false)).

2. Add symmetric moveForward()

Symfony's AbstractBrowser exposes both back() and forward() with identical semantics ($this->history->forward() + requestFromRequest($request, false)). InnerBrowser only had moveBack(). This adds the mirror moveForward(int $numberOfSteps = 1). It is only meaningful because the fix above keeps the forward stack intact:

$I->amOnPage('/a');
$I->amOnPage('/b');
$I->moveBack();
$I->seeCurrentUrlEquals('/a');
$I->moveForward();
$I->seeCurrentUrlEquals('/b');

Backward compatibility

No BC break:

• moveForward() — brand-new public method, purely additive.
• _loadPage() — gains a trailing optional parameter bool $changeHistory = true. Existing callers (incl. external Helpers using the @api method) are unaffected because the default preserves the previous behavior.
• moveBack() — signature unchanged; the change is a behavioral bugfix (back no longer mutates history). No relied-upon contract is removed.

Why it matters downstream

moveForward() and a correct moveBack() make the framework history assertions reachable through public steps — e.g. assertBrowserHistoryIsNotOnLastPage() (Symfony BrowserKitAssertionsTrait, symfony/browser-kit >= 7.4), which previously had no step to position history off the last page. Unblocks the matching functional tests in Codeception/symfony-module-tests (#58, #59) for codeception/module-symfony#240.

Tests

moveBack: added testMoveBackPreservesForwardHistory (fails on master, passes here).
moveForward: added testMoveForwardOneStep, testMoveForwardTwoSteps, testMoveForwardThrowsExceptionIfNumberOfStepsIsInvalid.
Full unit suite green: 209 tests, 469 assertions.