Add $plugin to the list of global variables that shouldn't be overridden

[johnbillion]

If a plugin overrides the $plugin global, things can break. See Rarst/laps#35.

This variable is missing from the list of global WP variables.

[jrfnl]

Verified and approved.

For future reference:
The variables is declared by WP in the global namespace in https://core.trac.wordpress.org/browser/trunk/src/wp-settings.php#L361-374 and unset() straight after the loop finishes.

A variable override of this variable will only cause a problem if the plugin being loaded declares the $plugin variable in the global namespace in the initial file being loaded - or in any plugin files being loaded directly when the initial file is read.

[Rarst]

Note that the same issue extends to $network_plugin and $mu_plugin. This is more noticeable now since starting with WP 5.1 these pass through new hooks, though they were never a good idea to override.

[jrfnl]

@Rarst Thanks for the heads-up. PRs for this are welcome 🤗