LibWeb: Implemented basic access_key_label function.

[bokendell]

This contribution is mainly due to a group project from the University of Utah CS4560 class.

The access_key_label function implementation was started and tested to provide the desired output. There are comments and debug lines included in the method, as we were not sure what the system modifier key was for serenity or how to get it per each OS. The same follows for determining a valid system key. The method defaults to providing "Alt+{access_key}" as the access_key_label and an empty string if not access_key is found.

Documentation followed: https://html.spec.whatwg.org/multipage/interaction.html#dom-accesskeylabel

[BuggieBot]

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[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!