Add notice if Xdebug extension is loaded

[aaemnnosttv]

As discovered in #1915 (comment), xdebug can cause AMP stylesheet parsing to take much longer than normal and potentially crash the site due to maximum execution limit timeout.

This PR adds a notice to inform the user of the potential risk.

I would expect perhaps the wording should maybe change, if so, just let me know!

Screenshot

[aaemnnosttv]

Updated per your feedback @westonruter.

[westonruter]

Looks good. I'll wait for a +1 from @schlessera before merging.

[ghost]

This should never have been added. If something in your plugin is making websites crash when xdebug is enabled, then surely shouldn’t the issue in your plugin be fixed rather than advising the user to disable the extension?

It’s not the users fault for having xdebug enabled, and most of the time it’s intentionally enabled so there should be no reason to force users to have to see a notice saying they’re doing something wrong.

There’s obviously an issue in the coding of your plugin that needs fixing.

[westonruter]

@leecollings if it bothers you so much, you can remove it yourself in another plugin with just this:

remove_action( 'admin_notices', '_amp_xdebug_admin_notice' );

[swissspidy]

I've created a new issue to discuss the placement of the warning: #3499.