[youtube] Match JSON in alert regex

[ZenulAbidin]

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• Bug fix
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Description of your pull request and other information

yt-alert-message div has moved to "alert" JSON key, so this PR adjusts the regex to match that.

I tested this change by dumping page into a file (such as this one) and manually editing the alerts[alertRenderer][text][run][text] subkey for each condition. Then you'd set a breakpoint at line 2894 or somewhere near that and load page using

with open('/tmp/yt-alert-message-div.txt') as ff:
    page = ff.read()

These steps allow you to reproduce these tests.