Ensure that the Application.start static method uses overridden class

[lb-]

• When using const application = MyApplication.start() - it now will use the actual MyApplication class instead of the non-extended Application class
• Fixes Ability to extend the Stimulus Application class #586

Notes

I opted for no documentation here - I think this behaviour would be expected and also aligns with any manual start usage with instantiation of the class yourself.

For example, the following two ways of starting the application will now be the same (with this PR merged).

A - Starting an application as per documentation

class MyApplication extends Application {}

const element = document.getElementById('main');
const application = MyApplication.start(element); // without this PR - the `application` will be an instance of the Stimulus `Application` and not `MyApplication`

B - Setting up the instance manually & running start

class MyApplication extends Application {}

const element = document.getElementById('main');
const application = new MyApplication(element);
application.start();

Test format

Note that I had to write my own test case class as src/tests/cases/application_test_case.ts does not use the documented Application.start() approach (due to other tests relying on a separate setup step for the application before starting).

This is similar to the way that the test file src/tests/modules/core/error_handler_tests.ts also sets up its own test case but without the double-registration of a Stimulus application. Happy to adjust if this is not the way we want to do this.

[lb-]

I do not think the error is due to this PR's changes (tests ran ok locally).

17 11 2022 21:36:45.021:ERROR [SaucelabsLauncher]: Error: 17 Nov 21:36:45 - couldn't connect to https://saucelabs.com/rest/v1/stimulus/tunnels: Post https://saucelabs.com/rest/v1/stimulus/tunnels: net/http: timeout awaiting response headers

[lb-]

Thanks @dhh