Introduce singular capabilities for managing individual plugins

[johnbillion]

In #38652, singular capabilities were added for activating and deactivating individual plugins.

The same should be added for other management actions for plugins:

• edit_plugin (ability to adit plugin via the Plugin Editor)
• delete_plugin (ability to outright delete the plugin)
• update_plugin (ability to update a plugin)

Network activation and deactivation will be handled in #42403

[antonioeatgoat]

Handled singular capability 'delete_plugin'. It still needs unit tests.

[antonioeatgoat]

Handled singular capability 'edit_plugin'. It still needs unit tests.

[antonioeatgoat]

Small fix for the 'delete_plugin' capability.

[antonioeatgoat]

Handled singular capability 'update_plugin'. It still needs unit tests.

[antonioeatgoat]

A quick clarification of the behavior of 'update_plugin'

[antonioeatgoat]

The implementation of 'update_plugin' has been more complex than the others, it would need to be tested further.

Also it needs a second opinion about the creation of the new function wp_get_plugin_updates_disallowed_count() (naming and behavior), and the general behavior (sorry for the typo):