Add `PluginPosition.None` to quickly hide a plugin

[mbolli]

Is your feature request related to a problem? Please describe.

Showing a plugin conditionally is already possible with PluginManager.remove() and PluginManager.add(). However, it is even easier to set Plugin.position to a non-existing PluginPosition. This works, but shows obviously a TypeScript warning TS2339: Property None does not exist on type typeof PluginPosition.

Describe the solution you'd like

To add PluginPosition.None to the Enum would not hurt anyone.

Describe alternatives you've considered

To ts-ignore my assignment line

Additional context

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