Update codebase to PHP 7.4

[TavoNiievez]

No description provided.

[ThomasLandauer]

You added the typehint string to the $selector argument of waitForText(): https://github.com/Codeception/module-webdriver/pull/82/files#diff-8885ce497c3c697e0101c0a22313db5a931cf71ad0a22a8d759ca724b2b1915aR2626
However, on https://codeception.com/docs/modules/WebDriver#Locating-Elements arrays are shown as example (e.g. ['id' => 'foo'])
Now my question is: Is this the first step to disallow arrays everywhere? Or was it just an error and the typehint should be changed to mixed?

Right now, I'm getting this error message:

[TypeError] App\Tests\AcceptanceTester::waitForText(): Argument #3 ($selector) must be of type ?string, array given

[TavoNiievez]

@ThomasLandauer it was an error, although I think it should be changed to string|string[], not mixed.

[ThomasLandauer]

Thanks, see #87